Handling Entity Framework Core database migrations in production – Part 2

Last Updated: July 31, 2020 | Created: January 30, 2019

This is the second in the series on migrating a database using Entity Framework Core (EF Core). This article looks at applying a migration to a database and follows on from part 1 which covered how to create a migration script. If you haven’t read the part 1 some parts of this article won’t make sense, so here is a very quick review of part 1.

  • There are two types of migrations that can be applied to a database:
    • Adds new tables, columns etc, known as a non-breaking change (easy).
    • Changes columns/tables and needs to copy data, known as a breaking change (hard).
  • There are two main ways to apply a migration to a database
    • Use EF Core migration feature
    • Use EF Core to create a migration and then hand-modify the migration.
    • Use a third-partly migration builder write in C# the migration.
    • Use a SQL database comparison tool to compare databases and output a SQL changes script.
    • Write your own SQL migration scripts by copying EF Core’s SQL.

So, now that you know how to create migration scripts now I’m going to look at the different ways you can apply a migration to a production database, with all the pros, cons and limitations.

TL;DR – summary of the content

NOTE: Click the links to go directly to the section covering that point.

Setting the scene – What sort of application have you got?

In part 1 we were focused on creating migrations that were “valid” and whether the migration is a non-breaking change or breaking change (see quick definition at start of this article, or this link for section in part 1).

Now we are looking at applying a migration to a database, but the options we have depends on the application (or applications) that are accessing the database. Here are questions you need to think about.

  1. Is there only one application accessing that database, or is your application a web app which is scaled-out, i.e. there are multiple versions of your application running at the same time. If your application is scaled-out, then this removes one of the options.
  2. Can you stop your application while you apply a migration to the database, or is your application providing a continuous (24/7) service? Updating continuous service applications bring some challenges when it comes to applying a breaking change.
When it comes to migrating a production database being a bit paranoid is OK.

As I said at the end of part 1 – the scary part comes when you apply a migration to a production database. Changing a database which contains business-critical data needs (demands!) careful planning and testing. You need to think about what you are going to do if (when!) a migration fails with an error.

When considering the different ways to apply a migration you should have in the back of your mind “what happens if there is an error?”. This might push you to a more complex migration approach because its easier to test or revert. I can’t give you rules or suggestions as each system is different but being a bit paranoid about failures isn’t a bad thing to have. I should make you build a system for migrating your application and its database that is more robust.

PART2: How to apply a migration to a database.

The list below gives the different ways you can apply a migration to a database. I list three options for the EF Core case: the first being the simplest, but it has limitations which the other two options don’t have. The SQL migration has no real limitations, but it does need a database migration application tool to apply the SQL scripts only once and in the right order.

Here is the list of ways you can apply a migration.

  1. EF Core migration
    1. Calling context.Database.Migrate() on startup
    2. Calling context.Database.Migrate() via a console app or admin command
    3. Outputting the migration as a SQL script and execute that script on the target database.
  2. SQL migrations
    1. Use a database migration application tool.

In the end, how you apply your migration depends on the type of migration (breaking or non-breaking) and the type of application you are updating (single app, multiple apps running in parallel or an app that mustn’t stop). Here is a diagram to try and convey all these permutations.

The outer dark blue shows that SQL migrations can be applied in all cases, then the lighter, inner boxes show where different types of EF Core migrations can be added. Here are some clarifying notes about the diagram:

  • The diagram shows standard EF migration and hand-modified EF migration, but when I am talking about applying the migration then there is no distinction between the two – we are simple applying an EF Core migration.
  • The “five-stage app update” red box in the diagram represents the complex set of stages you need to apply a breaking change to a application that cannot be stopped. I cover that at the end of the article.

Now I will go through each of the ways of applying a migration in detail.

1a. Calling context.Database.Migrate() on startup

This is by far the easiest way to apply a migration, but it has a big limitation – you should not run multiple instances of the Migrate method at the same time. That can happen if you scale-out a web application. To quote Andrew Lock, “It’s not guaranteed that this will cause you problems, but unless you’re extremely careful about ensuring idempotent updates and error-handling, you’re likely to get into a pickle” – see this section of his post “Running async tasks on app startup in ASP.NET Core”.

Good parts ·         It is relatively easy to implement (see tips)
·         It ensures the database is up to date before your application runs.
Bad parts ·         You must NOT run two or more Migrate methods in parallel.
·         If the migration has an error, then your application won’t be available.
·         It’s hard to diagnose startup errors
Limitations Does not work with continuous service systems
Tips I quite like this option in Andrew Lock’s article for running a migration on startup. I use a similar approach in some of my demo systems that use in-memory databases that need initializing (see this example)
My verdict If you are running a single web app or similar and you can update the system when no one is using it then this might work for you. I don’t use this as many of my systems I work on use scale-out.

1b. Calling context.Database.Migrate() via a console app or admin command

If you can’t run multiple Migrate methods in parallel, then one way to ensure this is to call the Migrate method inside a standalone application designed to just execute the Migrate method. You might add a console application project to your main web app solution which has access to the DbContext and can call Migrate. You can either run it yourself or let your deployment system run it (Note to EF6.x users – this the equivalent of running Migrate.exe, but with the application dll compiled in).

Good parts ·         It works in all situations.
·         Works well with deployment systems.
Bad parts A bit more work.
Limitations – none – , but watch out for continuous, five-stage app update
Tips If your console application takes in a connection string to define which database to apply the migration to, then it will be easier to use in your deployment pipeline.
My verdict A good option if you have a deployment pipeline, as you can execute the console application as part of the deployment. If you are manually applying the migration, then there is the command Update-Database.

1c. Turning EF Core migration into a script and applying it to the database

By using the Script-Migration command EF Core will convert a specific migration, or by default all your migrations, into a SQL script. You can then apply this using something that can execute SQL on the specific database you want updated. You can manually execute the SQL in SQL Server Management Studio, but typically you have something in your release pipeline to do that at the right time.

Good parts ·         It works in all situations.
·         Works well with deployment systems which can use SQL scripts.
·         You can look at the SQL before its run to see if it looks OK.
Bad parts ·         More work than the console app (1b)
·         You need some application to apply the script to the correct database.
Limitations – none – , but watch out for continuous, five-stage app update
Tips The SQL contains code to update the migration history, but you must include the idempotent option in the Script-Migration command to get the checks that stops a migration from being applied twice.
My verdict If you want to use EF Core’s Migrate method, then I would suggest using 1b, the console app. It’s as safe as using the scripts and does the same job. But if you pipeline already works with SQL change scripts then this is a good fit for you.

2a. Using a migration tool to apply a SQL script

If you create a series of SQL migrations scripts, then you need something to a) apply them in the right order and b) apply them only once. EF Core’s migrations contain code that implments the “right order” and “only once” rules, but when we write our own migration scripts we need a tool that will provides those features.

I, and many others, use an open-source library called DbUp that provides these features (and more) and also supports a range of database types. I order my migration scripts alphabetically, e.g. “Script0001 – initial migration”, “Script0002 – add seed data” for DbUp to apply. Just like EF Core migrations, DbUp uses a table to list what migrations have been applied to the database and will only apply a migration if it isn’t in that table.

Other migration tools are available, for instance Octopus Deploy, and various RedGate tools (but I haven’t used them so check they have the correct features).

Good parts ·         It works in all situations.
·         Works well with deployment systems.
Bad parts ·         You have to manage the scripts.
Limitations – none – , but watch out for continuous, five-stage app update
(for DbUp)
·         I make a console application that takes in the connection string and then runs DbUp, so I can use it in my deployment pipeline.
·         For testing I make the method that runs DbUp available to my unit test assembly in a “only run in debug mode” unit test that migrates my local database correctly using my CompareEfSql tool (see the section about testing migrations in part 1 of this series).
My verdict I use this approach on projects that use EF Core.

The application and applying migrations

When you apply a migration to the database you can stop the application or in some circumstances you can apply the migration while it is running. In this section I look at the different options available to you.

1. Stopping the application while you migrate the database

This is the safest option and works with breaking and non-breaking changes, but your users and your business might not be so happy. I call this the “site down for maintenance” approach. In the “site down” approach is you don’t want to stop an application while users are inputting data or maybe finished an order. That’s how you or your company gets a bad reputation.

I had this problem myself back in 2015 and I created a way to warn people that the site was going to close and then stopped all but the admin person from accessing the application. I chose this approach because for the web application I was working on it was a less costly approach than supporting breaking changes while keeping the web app running (I cover applying breaking to a continuous service application later). You may have come across “this site is down for maintenance” on services you use, normally at weekends and overnight.

NOTE: I wrote an article called How to take an ASP.NET MVC web site “Down for maintenance” which you might like to look at – the code was for ASP.NET MVC5 so it will need some work to get it to work with .NET Core, but the idea is still valid.

Applying non-breaking migrations while the application is running

With non-breaking changes you can, in theory, apply them to the database while the old application is running, but there are some issues that can catch you out. For instance, if you added a new, non-null column with no SQL default and old software, which doesn’t know about that new column, tries to INSERT a new row you will get a SQL error because the old software hasn’t provided a value for a non-null column.

But if you know your non-breaking migration doesn’t have a problem then applying the migration while the old application is running provides continuous service to your users. There are various ways to do this, depending on which of the migration application approach you have chosen, one that come to mind are Azure’s staging slots, which have been around for ages, and the newer Azure Pipelines.

Applying breaking changes to a continuous running application: The five-stage app update.

The hardest job is applying a breaking change to a continuously running application. In the diagram showing the different approaches to will see a red box called “five-stage app update” in the top-right. The name comes from the fact that you need to migrate in stages, typically five, as shown in the diagram below.

NOTE: Andrew Lock commended that my “add a non-nullable column” problem I described in the last section can be handled in three stages: a) add new column but as nullable, b) deploy new software that knowns about that column, and c) alter the column to be non-nullable.

Here is a diagram taken from in section 11.5.3 of my book “Entity Framework Core in Action” which shows the five stages needed to add a breaking change that split an existing CustomerAndAddress table into two tables, Customers and Addresses.

As you can see an update like this is complex to create and complex to apply, but that’s the cost of running a continuous system. There aren’t any real alternatives to the five stages, other than you never apply a breaking change to a continuous running system (I have heard one person who said that was their approach).

NOTE: I cover the continuous, five-stage app update in section 11.5.3 on my book “Entity Framework Core in Action” and you can also find a coverage of this in chapter 5 of the book “Building Evolutionary Architectures” by Neil Ford et al.


If the data in your database and the availability of your service is important to your organisation, then you must take a database migration seriously. In part 1 I covered the different ways create a migration script and this article covers how you might apply those migrations to a production database. The aim of this series is to provide you with the options open to you, with their pros, cons and limitations, so that you can take an informed decision about how to handle migrations.

As I said in the first article my first run-ins with EF migrations was with EF6. I know EF6 very well and having written the book “Entity Framework Core in Action” I know EF Core even better. The change from EF6 to EF Core around migrations typifies the change in the whole approach in EF Core.

EF6 had lots of “magic” going on to make it easier to use – automatic migration on startup was one of them. The problem was, when EF6’s “magic” didn’t quite work, then it was hard to sort it out. EF Core’s approach to migrations is that its up to you where and how you use it – no automatic “magic”. And lots of other small changes to migrations in EF Core come from listening to users of EF4 to 6.

So, migrations on production database is scary, but its always been scary. I have given you some insights into the options but that’s only really the minimum for production database changing. Backups, policies, pre-prod testing and deployment pipelines need to be added as required to make a reliable system.

Happy coding.

Handling Entity Framework Core database migrations in production – Part 1

Last Updated: July 31, 2020 | Created: January 29, 2019

Andrew Lock wrote an excellent series called “Running async tasks on app startup in ASP.NET Core” in which he used “migrating a database” as an example of something you could do on startup. In part 3 of the series he covered why migrating a database on startup isn’t always the best choice. I decided to write a series about the different ways you can safely migrate a database, i.e. change the database’s schema, using Entity Framework Core (EF Core).

This is the first part of the series, which looks how to create a migration, while part 2 deals how to apply a migration to a database, specifically a production database. I have used a similar approach to Andrew’s when writing this article, i.e. I try to cover all the way to create a migration script when working with EF Core with their pros, cons and limitations.

NOTE: Andrew and I know each other as we were both writing books for Manning Publications at the same time: Andrew’s book is “ASP.NET Core in Action” and mine is “Entity Framework Core in Action”. We shared the toils and joys of being an author, but Andrew has the harder job with ASP.NET Core – his book is 700 pages long and mine is “only” 500 pages long.

TL;DR – summary of creating a migration

NOTE: Click the links to go directly to the section covering that point.

Setting the scene – what questions should we ask about creating a migration?

There are lots of ways to migrate a database’s schema, and in development you can use almost any approach. But when it comes to migrating a production database (i.e. the one that is being used by real users) then it becomes very serious. Getting it wrong will, at the minimum, inconvenience your users and at worse mangle or lose you (precious) data in your database!

But before we get the scary part of updating a database schema we need to build the migration script that will contain the schema, and possibly data, changes. To build the appropriate migration script we need to ask ourselves some important questions about the type of changes we need to apply to the database. Is the required migrations going to be:

  1. A non-breaking change, i.e. it only adds new columns, tables, etc., which could be applied while the old software was still running, i.e. the old software would work (not break) with the database after the migration.
  2. A breaking change, i.e. some data must be copies or transformed during the migration, could NOT be applied while the old software, i.e. the old software would encounter errors (break) with the database after the migration.

The other part in this article is that we are working with EF Core, which brings both some benefits and constraints. The benefits are that EF Core can, in most cases, create the required migrations automatically. The constraints are that the database after a migration has been applied must match the software Model of the database that EF Core builds by looking at your DbContext and mapped classes – I refer to EF Core’s Model with a capital M, because there is a property called Model in the DbContext that contains the complete mappings between classes and the database.

NOTE: I’m going to cover migrations where you are in control of the control of the classes mapped to the database and the EF Core configuration – sometimes known as the Code-First Approach. The other alternative, which I’m NOT going to cover, is where you directly control the database and you use an EF Core command called scaffolding to create the entity classes and EF Core configuration for you. That’s because migrations are trivial – just re-scaffold your database.

PART1. The five ways to create a migration script

As I said in the last section, any migration script we create must migrate the database to a state that matches EF Core Model. For instance, if a migration added a new column to a table then the entity class mapped to that table must have a property to match that new column. If EF Core’s Model of the database schema does match the database, then you can get errors occurring in queries or writes. I refer to the migration script as creating a “valid” database if it matches EF Core’s Model of that database.

The validity of a migrations created by EF Core isn’t in doubt – EF Core created it so it will be valid. But if we need to edit a migration, or we take on the job of building the migration ourselves, then we need to be very careful that the migration creates a “valid” database as far as EF Core is concerned. This is something I have thought about a lot.

This is the list of ways you can create a migration script.

  • Creating C# migration script
    1. Standard EF Core migration script: use EF Core’s Add-Migration command to create a C# migration script.
    2. Hand-modified EF Core migration script: Use EF Core’s Add-Migration command to create a C# migration script and then hand-edit it to add the bits that EF Core missed.
    3. Use a third-partly migration builder, e.g. FluentMigrator. Tools like this allow you to write your own migration script in C#.
  • Creating SQL migration script.
    1. Use a SQL database comparison tool. This compares the last database schema with the schema of a new database created by EF Core and produces a SQL script that will migrate the old database to the new database schema.
    2. Write your own SQL migration scripts. A competent writer of SQL could produce a SQL migration scripts, with some help by capturing the SQL EF Core would use to create a database.

Here is a summary diagram to give you an overall review of the five approaches, with my personal view on the ease of use and level of limitations.

Now let’s look at each one of these in turn.

1a. Standard EF Core C# migration script

This is the standard migration technique provided by EF Core. It’s well documented by Microsoft, but in summary you run a command called Add-Migration which adds three C# files to your application that contain the changes needed to migrate an existing database using migraions to match the current EF Core setup/configuration.

Good parts ·         Builds a migration automatically
·         You don’t need to learn SQL
·         Includes a revert migration feature
Bad parts
Limitations ·         Standard migration cannot handle breaking changes (but see 1b).
·         No handling of SQL features, e.g. SQL user defined functions (but see 1b).
Tips ·         Watch out for error messages when you run the Add-Migration method. If EF Core detects a change that could lose data it will output an error message, but still creates the migration files. YOU MUST alter the migration script otherwise you will lose data – see section 1b.
·         If your DbContext is in another assembly from where the DbContext is registered you need the MigrationsAssembly method on your build and you most likely you need to implement a IDesignTimeDbContextFactory in the DbContext assembly.
My verdict This is a really easy way to handle migrations and it works well in many cases. The problem is, what happens if the migration doesn’t cover what you want. Thankfully there are many ways to handle that.

Reference: Microsoft’s documentation on creating a migration.

1b. Hand-modified EF Core C# migration script

The nice thing about EF Core’s Add-Migration command is it creates the C# migration files as the starting point, but you can then edit these files yourself to add code to handle breaking changes or add/update SQL parts of the database. Microsoft give an example of handling a breaking change with copying data.

Good parts Same as standard migration +
·         Ability to customise the migration.
·         Ability to include SQL features, e.g. SQL user defined functions.
Bad parts ·         You need to understand what is happing in the database.
·         Can be difficult to decide on how to edit the file, e.g. do you keep everything EF Core added and then alter it, or remove EF Core parts and do it yourself?
Limitations No simple way to check the migration is correct (but see CompareEfSql later).
Tips Same as standard migration.
My verdict Great for small alterations, but big changes can be hard work as you are often mixing C# commands with SQL. That is one reason why I don’t use EF Core migrations.

Reference: Microsoft’s example of hand-modifying a migration.

1c. Use a third-partly C# migration builder

It was Andrew Lock who pointed out to me an approach using the FluentMigrator (see this documentation) to write your migrations. This works similarly to EF migrations, but you have to do all the hard work of detailing the changes. The good thing is FluentMigrator’s commands are very obvious.

Good parts ·         Don’t need to learn SQL.
·         Very obvious what the changes are, i.e. “code as documentation”.
Bad parts ·         You must work out what the changes are yourself.
·         Not guaranteed to produce a “correct” migration (but see CompareEfSql later).
Limitations – none –
Tips Note that FluentMigrator has a “Migration Runners” which can apply the update to the database, but it can also output SQL scripts instead.
My verdict No real experience myself. It feels like it is a clearer syntax that EF Core’s migration, but you have to do all the work yourself.

Reference: FluentMigrator of GitHub.

2a. Use SQL database comparison tool

There are free and commercial tools that will compare two databases and create a SQL change script that will migrate the old database schema to the new database schema.

A “free” comparison tool is built into Visual Studio 2017 (all versions) called SQL Server Object Explorer, under the View tab. If you right-click on a database, then you can access the “Compare Schema” tool (see figure above right) which can produce a SQL change script.

The SQL Server Object Explorer tool is very good, but there isn’t much documentation on this (pity). Other commercial systems include Redgate’s SQL Compare.

Good parts Builds the correct SQL migration script for you.
Bad parts ·         You need a little bit of understanding of databases.
·         No all SQL compare tools produce a revert script.
Limitations Does not handle breaking changes – needs human input.
Tips Watch out for SQL compare tools that outputs every setting under the sun to make sure it gets it right. EF Core’s migrations are straightforward, like “CREATE TABLE…”, so should do that same. If you have any specific settings, then build them into the database create.
My verdict I have used SQL Server Object Explorer on a big migration that was hard to hand-code. Very useful and especially good for people who aren’t comfortable with the SQL language.

2b. Hand-coding SQL migration scripts

This sounds really hard – writing your own SQL migrations, but there is plenty of help on hand, either from SQL compare tools (see above) or by looking at the SQL EF Core would use to create the database. This means I have SQL I can look at and copy to build my SQL migration script.

Good parts Total control over the database structure, including parts that EF Core won’t add, e.g. user defined functions, column constraints, etc.
Bad parts ·         You must understand basic SQL like CREATE TABLE etc.
·         You must work out what the changes are yourself (but there is help)
·         No automatic revert migration.
·         Not guaranteed to produce a “correct” migration (but see CompareEfSql later).
Limitations – none –
Tips ·         I use a unit test that captures the logging output of EF Core’s EnsureCreated method. That gets me the actual SQL EF Core would output. I then look for the differences for the last database. That makes writing the SQL migrations much easier.
·         I unit test a migration by creating a database by applying all the migrations, including the new migration, and then running CompareEfSql to check the database matches EF Core’s current Model of the database.
My verdict This is what I use, with a little help from a CompareEfSql tool. If EF Core’s migration feature is so good, why do I go to all this trouble? Here are my reasons:
·         Total control over the database structure, including parts that EF Core won’t add, e.g. user defined functions, column constraints, etc.
·         Because I am writing the SQL it makes me think about the database aspects of the change – should that property be nullable? do I need an index? etc.
·         Handing breaking changes by hand-modifying EF Core’s migration system isn’t that easy. I might as well stick with SQL migrations. 

This is for developers that wants total control and visibility of the migrations.

You can capture the SQL output by EF Core’s for creating a database but capturing the EF Core’s logging while calling the method EnsureCreated (the EnsureCreated method is meant for creating unit test database). Because setting up logging for EF Core is a little bit complex I added helper methods to my EfCore.TestSupport library to handle that. Here is an example unit test that creates a new SQL database and catches the SQL commands that EF Core produces.

public void CaptureSqlEfCoreCreatesDatabaseToConsole()
    var options = this.CreateUniqueClassOptionsWithLogging<BookContext>(
        log => _output.WriteLine(log.Message));
    using (var context = new BookContext(options))


Let’s look at each line of this code

  • Line 5. This is an EfCore.TestSupport method that creates the options for your DbContext. This version uses a database name that includes the class name. I do this because xUnit test classes are run in parallel, so I want a unique database for this unit test class.
  • Line 6. I use the version of the option builder ending with …WithLogging, which allows me to capture the log outputs. In this case I output the Message part of the log directly to the unit test output window.
  • Lines 11 & 12. First, I ensure the database is deleted so that when I call EnsureCreated a fresh database will be created with a schema defined by the current DbContext’s configuration and mapped classes.

Below is part of the output captured in the unit test output. This provides you with the exact SQL that EF Core would use to create the whole schema. You do need to extract just the parts that relate to your migration, but at least you can cut-and-paste the parts you need into your SQL migration script.

CREATE DATABASE [EfCore.TestSupport-Test_TestEfLogging];
Executed DbCommand (52ms) [Parameters=[], CommandType='Text', CommandTimeout='60']
IF SERVERPROPERTY('EngineEdition') <> 5
Executed DbCommand (5ms) [Parameters=[], CommandType='Text', CommandTimeout='30']
CREATE TABLE [Authors] (
    [AuthorId] int NOT NULL IDENTITY,
    [Name] nvarchar(100) NOT NULL,
    CONSTRAINT [PK_Authors] PRIMARY KEY ([AuthorId])
Executed DbCommand (1ms) [Parameters=[], CommandType='Text', CommandTimeout='30']
    [BookId] int NOT NULL IDENTITY,
    [Title] nvarchar(256) NOT NULL,
-- rest of SQL left out

How to be sure your migration is valid – use CompareEfSql tool

I have mentioned CompareEfSql a few times in the descriptions of creating migrations. This tool compares a database with the model of the database that EF Core creates on first use for a DbContext. This model, accessed via the Model property in the DbContext instance, is built up my EF Core by looking at the DbContext configurations and DbSet and DbQuery properties.

This allows a developer to test an existing database against EF Core Model and gives you error messages if they are different. I find this a very powerful tool which allows me to hand-code SQL migrations and be sure that they are correct (there are some small limitations). Here is an example unit test that will fail if the database schema doesn’t match EF Core’s Model.

public void CompareViaContext()
    var options = … options that point to the database to check;
    using (var context = new BookContext(options))
        var comparer = new CompareEfSql();

        //This will compare EF Core model of the database 
        //with the database that the context's connection points to
        var hasErrors = comparer.CompareEfWithDb(context);

        //The CompareEfWithDb method returns true if there were errors.
        //The comparer.GetAllErrors property returns a string
        //where each error is on a separate line

I love this tool, which is in my EFCore.TestSupport open-source library. It allows me to build migrations and be sure they are going to work. I also run it as a normal unit test and it tells me immediately if I, or another team mate, has changed EF Core’s setup.

You can get a much longer description of this tool in the article called EF Core: taking full control of the database schema and its many features and configurations can be found in the CompareEfSql documentation pages.

NOTE: I build a version of this originally for EF6.x (see this old article), but it was limited because EF6.x didn’t fully expose its internal model. With EF Core I could do so much more (EF Core rocks!) and now I can check almost everything, and because I tap into EF Core’s scaffolder service it works for any database that EF Core supports (see this doc).

Conclusion – Part 1

This part of the series covers creating a valid migration, while part 2 deals with applying a migration to a database. This article lists all the applicable approaches to creating a database migration when working with EF Core – with their pros, cons and limitations. And as you can see EF Core’s Add-Migration command is really good, but it doesn’t cover every situation.

Its up to you to decide what types of migrations you might encounter, and what level of control do you want over the database schema. If you can get away with just using EF Core’s standard migrations (1a) then that makes life easier for you. But if you expect breaking-changes, or you need to set up extra SQL features then you now know the options available to you.

The scary part comes in the part2 – applying a migration to a production database. Changing a database which contains business-critical data needs (demands!) careful planning and testing. You need to think about what you are going to do if (when!) a migration fails with an error.

The original reason I moved away from EF migrations in EF6 was its automatic migration on startup was working fine, until it threw an error on deployment! It was really hard to track down an error in the migration – that alone got me moving away from using EF migrations (read this old article on my thoughts back then).

EF Core’s migration handling is better than EF6: automatic migrations have (thankfully!) gone and EF Core migration are more git-merge friendly, to mention just two changes. But the way I build SQL migration scripts makes me think much more carefully about what I am doing than just running Add-Migration. EF Core is a really great O/RM but it does hide the database too well sometime. Creating SQL migration scripts makes me think through a migration from the database point of view, and I often spot little tweaks to the database, and the C# code, to make the database better or more robust.

Now go to part 2 to see how to apply a migration to a database.

Wrapping your business logic with anti-corruption layers – NET Core

Last Updated: July 31, 2020 | Created: January 21, 2019

There is a concept in Domain-Drive Design (DDD) called the anti-corruption layer which, according to  Microsoft explanation of an anti-corruption layer “Implements a façade or adapter layer between different subsystems that don’t share the same semantics”. DDD uses anti-corruption layers between subsystems (known in DDD as a bounded contexts), say inventory, orders, fulfilment/delivery in an e-commerce system.

But I think the anti-corruption layer concept can also help us inside even small applications. In fact, I expect many of you are already the anti-corruption layer concept in your application, but you call them “adapters”, or “interfaces”, or “ViewModels”.

In this article I’m going to explore the application of the anti-corruption layer not to big subsystems, but to individual NET assemblies in even a small web application, e.g. between the database, the business logic, and the front-end. I hope to show how to implement anti-corruption layers and why they will make your applications easier to write and maintain.

This article was inspired by a problem I encountered in the anti-corruption feature in my EfCore.GenericBizRunner library. I had configured the DTOs (data transfer objects) with the AutoMapper library (which I use for class-to-class mapping) in such a way that some AutoMapper config properties were turning up in my output classes/DTOs. That can cause a bit of a problem when using DTOs in Web APIs, so I fixed it in version 3 (out now).

Other articles about business logic and GenericBizRunner:

The examples I am using are based around ASP.NET Core web applications and Entity Framework Core (EF Core) for database accesses.

TL;DR; – summary

  • DDD defines an anti-corruption layer is an adapter pattern that isolates one part of a system, known in DDD as a bounded context, from another bounded context. Its job is to ensure that the semantics in one bounded concept do not “corrupt” the other bounded concept’s semantics.
  • I think that the anti-corruption layer concept can also be used between assemblies even in small application. Some examples of using an anti-corruption layer in a web application are:
    • Business logic: Your business logic shouldn’t have to concern itself about how the database or the front-end works.
    • Front-end: Human user’s need the data that suits them, not how the database was built.
    • Web API: Your Web API must provide a service that fits the external needs.
  • Anti-corruption layers can help with security too, by only passing the exact data that the other area needs.

Business logic – the classic area for anti-corruption layers

In the book Domain-Driven Design, Eric Evan that “Domain Model” (what I call “business logic”) is  “the heart of the Software” (see Eric Evans book, page 4). Eric goes on to say “When the domain is complex, this is a difficult task, calling for the concentrated effort of talented ad skilled people”.

I wrote an article called “Architecture of Business Layer working with Entity Framework” where I go through a design for handling business logic. That article is worth a read, but in this article I focus on the anti-corruption layers as applied to my business logic.

From Eric Evan’s book, and my own experience, I want to have no distractions when I’m writing my business logic. That translates into the following “don’t wants”:

  • I don’t want to worry about how the database works.
  • I don’t want to worry about how my business logic communicates to a user.
  • I don’t want to know I am using an external service – I just want methods to call.

How I achieve this isolation differs for each part: in this case I use a repository pattern for the database side and my GenericBizRunner library to talk to the front-end. I will describe these two anti-corruption layer implementations later, but first here is a diagram give you an overview of my approach.

I’m now going to describe each of these two anti-corruption layers, i.e.

  1. The database to business logic anti-corruption layer.
  2. The business logic to front-end anti-corruption layer.

1. Implementing the database to business logic anti-corruption layer

Eric Evans talks in his book about separating the business logic from the database and EF Core’s navigational properties makes this possible.  The navigational properties allow the business logic to work with normal (POCO) classes that have properties that references other classes. That means the business logic doesn’t have to know about primary keys, foreign keys, etc.

Let’s look at a specific piece of business logic, placing an order for some books, to show this in action.  The user selects a series of books (front-end lineItems) and the business logic creates an Order containing database LineItems. To do this my business logic, called PalceOrder, has to take in the front-end lineitems and create a new Order class and adds a series of database LineItem classes to it.

That’s fine, but it does need to access the database for two things:

  1. The front-end lineItems contain the primary key of each book, but the database LineItem class needs the Book entity class to create it. This needs a database lookup.
  2. The PlaceOrder code creates a new Order class, but somehow that class needs to be added to the database.

My solution is to use a repository pattern to provide the business logic with methods to handle these two database requirements.  Here is the code, which the business logic accesses via constructor injection via the IPlaceOrderDbAccess interface.

public class PlaceOrderDbAccess : IPlaceOrderDbAccess
    private readonly EfCoreContext _context;

    public PlaceOrderDbAccess(EfCoreContext context)
        _context = context;

    public LineItem BuildLineItem(int bookId, byte numBooks)
        return new LineItem(bookId, _context.Find<Book>(bookId), numBooks);

    public void Add(Order newOrder)                 

This repository is small because it only implements database access methods for just the PlaceOrder business logic. A typical repository has tens or hundreds of methods to be used all over the application, but I don’t find that that approach very useful. I have applied the single responsibility principle to the repository pattern and come up with a per-business logic repository, which I call a “mini-repository”.

The benefit of the mini-repository pattern is that the code is much simpler to write and maintain. It also means later changes in architecture, such as splitting a monolith architecture up into microservices or serverless, is easier to do as the business logic and the associated mini-repository can move together.

NOTE: I haven’t talked about saving the data to the database – in EF Core that means calling the method called SaveChanges. I could have included that in my mini-repository (most likely in the Add method), but my GenericBizRunner library handles calling the SaveChanges method. That allows the library to add validation of the entities to be saved, which I think is important for business logic.

2. Implementing the business logic to front-end anti-corruption layer

The business logic and the front-end have a common goal: to place an order for books. But both parts have their own concepts – the business logic is about applying a business rule while the front-end wants to provide an easy-to-use user interface. So, in fact the anti-corruption cuts both ways – stopping concepts/data that are useful in one assembly from leaking into the other assembly.

Here is a diagram that shows the various NET assemblies and what happens in each stage of the process of running some business logic that accesses the database. It’s quite detailed, but it conveys in one picture all the main things that are going on. I include a commentary on what my GenericBizRunner library is doing – you could of course write your own code to handle these jobs.

NOTE: The Service Layer is an assembly that I use for adapting data from lower layers to the front-end. I got this concept from Dino Esposito (who got it from Martin Fowler). You can get more information in the section on the Service Layer in my article “Six ways to build better Entity Framework (Core and EF6) applications”.

2a. front-end to business logic anti-corruption layer

The front-end might need all sorts of data to show a page to the user, but the business logic only wants the list of books with their unique IDs (primary key in this case). In ASP.NET the HTML page often sends back extra data in case there is an error and the needs to be redisplay. But the business logic doesn’t need that data, and in some case that data is in NET Type that the business logic doesn’t even know about, i.e. a SelectList type. How do we handle that?

The way I handle that is to use AutoMapper in my library to copy the front-end ViewModel/DTO into the input class that my business logic needs. Here is an example from another piece of business logic to change the delivery date of an order. The diagram below displays the front-end focused DTO and the business focused input DTO.

You can see that the front-end needs extra information to show the user what order they are changing (left-hand side of diagram) while on the right-hand side the business logic only needs the first three properties. I (or to be more correct my GenericBizRunner library) uses AutoMapper to copy the data across before the business logic is called.

I think this is a very good example of an anti-corruption process going on. The front-end has extra properties to display a meaningful page to the user, but the business logic doesn’t even know about these front-end only properties. Also, the anti-corruption layer means that the business layer doesn’t have to know about or support front-end types such as SelectList etc.

My GenericBizRunner has a few features to help in a case like this. It provides the front-end DTO with a method called SetupSecondaryData, which can be run to fill in extra data needed for the display to the user – in this case filling the extra properties to show or re-show the display. Here is the code in the ASP.NET Core MVC controller to display and ChangeDelivery page and called the business logic process the user’s request.

public IActionResult ChangeDelivery(int id, 
    [FromServices]IActionService<IChangeDeliverAction> service)
    //This runs SetupSecondaryData to fill in dropdownlist etc.
    var dto = service.GetDto<WebChangeDeliveryDto>(x =>
        x.OrderId = id;
        x.UserId = GetUserId(HttpContext);
    return View(dto);

public IActionResult ChangeDelivery(WebChangeDeliveryDto dto,
    [FromServices]IActionService<IChangeDeliverAction> service)
    if (!ModelState.IsValid)
        //Rerun SetupSecondaryData if need to redisplay
        return View(dto);


    if (!service.Status.HasErrors)
        return RedirectToAction("ConfirmOrder", "Orders", 
            new { dto.OrderId, message = service.Status.Message });

    //Otherwise copy errors in and redisplay the page 
    service.Status.CopyErrorsToModelState(ModelState, dto);
    return View(dto); //redisplay the page, with the errors

You can see on lines 2 and 16 that I inject an instance of the GenericBizRunner, with the specific business logic, referred to via the interface IChangeDeliverAction, as a parameter to the action. This is more efficient than using the normal constructor DI injection as we only inject the exact service we need for each action method.

2b. business logic to front-end anti-corruption layer

Sometimes its also useful to not send everything that the business logic sends back. In the PlaceOrder example the business logic works with classes and (in theory) shouldn’t need to think what the front-end needs. Therefore, the PlaceOrder business logic will return the Order class.

The problem is that Order class might have all sorts of data that is private, like information on the payment, but the front-end only wants to have the primary key of the Order so that it can display an “order success” page. The solution is the same as the input: only copy over the properties that the front-end needs – in this case only copy the OrderId from the Order class that the business logic returns, but only after SaveChanges has been called and the primary key is available.

To make this work I need two things:

  1. An OrderIdDto class with just one property in it called OrderId, which should hold the primary key of the created Order. This is the only bit of data that my front-end needs.
  2. Once SaveChanges has been called I copy over the OrderId property in the Order into the OrderIdDto class’s OrderId property.

Here is the controller action that calls PlaceOrder. Note that the user’s basket of items is held in a cookie, so you will see some code to get and clear the checkout cookie.

public IActionResult PlaceOrder(PlaceOrderInDto dto, 
    [FromServices]IActionService<IPlaceOrderAction> service)
    if (!ModelState.IsValid)
        //model errors so return to checkout page, showing the basket
        return View("Index", FormCheckoutDtoFromCookie(HttpContext));

    //This runs the PlaceOrder business logic
    var orderDto = service.RunBizAction<OrderIdDto>(dto);

    if (!service.Status.HasErrors)
        //If successful I need to clear the checkout cookie
        return RedirectToAction("ConfirmOrder", "Orders", 
            new { orderDto.OrderId, Message = "Your order is confirmed" });

    //There were errors so redisplay the basket from the cookie
    var checkoutDto = FormCheckoutDtoFromCookie(HttpContext);      
    //This copies the errors to the ModelState
    service.Status.CopyErrorsToModelState(ModelState, checkoutDto);
    return View("Index", checkoutDto);

You can see the GenericBizRunner’s call the PlaceOrder’s method on line 13, via the service injected as a parameter. In that call I tell it that the return type should be OrderIdDto, which I have created as a DTO type that GenericBizRunner knows needs to be mapped from the Order class. This means only primary key of the order ends up in the front-end and all the other properties in the Order class is discarded.  The front-end can then pass on this primary key to the ConfirmOrder page which knows how to display a successful order.

GenericBizRunner and Web APIs

As I said at the start I found a problem that placed unwanted properties in the DTOs. To me it matters a lot that I get the data returned from the business logic in the exact format needed by the Web API. This really mattered on one of my client’s project which used Swagger and NSwagStudio to create Angular 6 code to match the Web API. NSwagStudio was amazingly useful, as it automates the creation of the Angular code to interface to the Web APIs, BUT you must get exactly the return type to make it work property.

Now, is this an anti-corruption feature, or just the normal adaption that we are already used to? I think adaption of data is what is mainly going on, but thinking “what anti-corruption issues are there?” is useful. Here are my thoughts:

  • Anti-corruption says we need to only pass the data that is required by the Web API definition.
  • Anti-corruption makes us thing about the data we do pass: is it in the correct form, are the names of the objects right for the Web API, etc.

A look at GenericBizRunner in a controller

With the help of my companion EfCore.GenericServices.AspNetCore library using GenericBizRunner in Web API is pretty simple. Here is a simple example taken from the EfCore.GenericServices.AspNetCore’s ExampleWebAPI. It uses some business logic to create a TodoItem.

[ProducesResponseType(typeof (TodoItem), 201)]
public ActionResult<TodoItem> Post(CreateTodoDto item, 
    [FromServices]IActionService<ICreateTodoBizLogic> service)
    var result = service.RunBizAction<TodoItem>(item);
    return service.Status.Response(this, 
        "GetSingleTodo", new { id = result?.Id }, result);

Let’s look at each line of this code

  • Line 1: I am using Swagger, and I want to tell Swagger a) the type of result it provides, and b) the status code on success.
  • Line 3: It pass in a CreateTodoDto, which contains the properties that the business logic needs to create the TodoItem.
  • Line 4: This is where I inject the BizRunner instance, linked via the interface to the business logic to run.
  • Line 6: I run the business logic, which returns the created TodoItem instance.
  • Lines 7 and 8: The Response method is from the EfCore.GenericServices.AspNetCore library. This form returns a “CreatedAsRoute” result, which fits the definition of the HTTP 201 create response.

In this case I have returned the created class, but I have found in real applications you really don’t want to do that. It turns out that EF Core’ entity classes very often have navigational properties. These are properties that link to other entity classes in the database, e.g. a Book has a collection of Reviews.

The problem is that many SPA’s (single page applications, like Angular and React.js) only want the non-navigational data. In these cases, we need to use DTOs that remove those navigational properties and/or applying some form of transformation to the data when sending database classes to the Web API. Thankfully the GenericBizRunner can do that.


Well, I hope I have made you think about the anti-corruption concept and whether it would improve your business logic. You are most likely doing something similar with adapter patterns and ViewModels/DTO, but you might not call it an anti-corruption layer. That’s fine, but thinking “anti-corruption” can help you implement a good pattern for your business logic.

The down-side of the approach I have described is it does create more code. For small bit of business logic then it might feel like overkill, but when the business logic gets more complicated then I think some separation of concerns pattern is essential. The pattern I have described is one way, but there are plenty of other approaches that achieve the same result.

I created the EfCore.GenericBizRunner library to reduce the amount of extra code I must write, especially in the front-end where it makes it very easy to call. Also, having a standard pattern for my business logic helps me write my business code quickly. I do have to write the mini-repository as well, but that database access code had to go somewhere and my mini-repository per business logic makes it easy to change or performance tune. So far this approach and the GenericBizRunner library has served me well.

Happy coding.

Part 2: Handling data authorization in ASP.NET Core and Entity Framework Core

Last Updated: July 31, 2020 | Created: January 14, 2019

In the first part of this series I looked at authorization in ASP.NET Core, but I only focused on controlling what pages/features the logged in user can access. In this article I’m going to cover how to control what data a user can see and change. By data I mean dynamic information that is stored in a database, such as your home address, what orders you made on an e-commerce site etc. Lots of web sites need to protect their, and your, data – it’s a hot topic now and you really don’t want to get it wrong.

This article focuses on controlling access to data in an ASP.NET Core application using Entity Framework Core (EF Core) to access the database. I have extended the example ASP.NET Core application, I built for the first article and added a new ASP.NET Core MVC web app called DataAuthWebApp  which covers data authorization instead of the feature authentication I have already described in Part 1.  This is an open-source (MIT licence) you can look at to see how it works underneath.

UPDATE: New articles added to this series

There was a lot of interest in these articles so I have written some follow-on articles that answer some of the comments/questions from the first two articles, plus a new, improved example application.

The Part 4 article is worth reading after this article, as it goes much deeper into the design of the data authorisation parts.

NOTE: you can Clone the GitHub repo and run locally – it uses in-memory databases so it will run anywhere and I seed it with test data. The application was written using ASP.NET Core 2.1 and EF Core 2.1: parts of the ASP.NET Identity is changing, but the overall concept will work with any version of ASP.NET Core.

This is a long article, so here are links to the major parts:

TL;DR; – summary

  • You can control what data a user sees by adding [Authorize(…] attributes to your controller actions or configuring razor pages, but it’s an all-or-nothing control – see the Part1 article for how to do this.
  • If you want to filter the rows in a table, say only returning data associated to the current logged-in user, the you need a per-row data protection system – see this article on how to do that.
  • The best way to implement per-row data protection is to put all the code inside EF Core’s DbContext. That way the protection is on be default to every class/table that you want protected.
  • The process to add per-row protection to an entity class (table) is
    • Mark every new row added to a table with a protect key, and…
    • All queries for that entity class/table are filtered using a key provided from the user’s information.
  • This article starts with the protection of personal data, then moves onto the handling grouped user access (known as multi-tenant systems) and finishes with the more complex hierarchical (e.g. manager->staff) system example.
  • All the code in this article is available as an open-source repo on GitHub – see PermissionAccessControl repo.

Setting the Scene – the different ways for protecting data

An ASP.NET Core web application typically has two parts:

  • Static parts that is fixed by the code, e.g. how a page looks or the type of Web API it provides. Typically, you need to update your code and publish it to change the static parts.
  • Dynamic data that can change, e.g. data held in a database or data obtained from external services.

In this article I’m going to look how you can protect data in a database. When we say we want to protect data we are talking about controlling who can see, or change, data stored in the database. If going to discuss two forms of data protection: all-or-nothing and per-row protection – see diagram below

The all-or-nothing data protection is where the user can either access all of the data or can’t access it at all. In ASP.NET applications this is done by placing authorization controls on the controller actions or Razor Pages. This does allow separate control over different types of access to the data, for instance, we can allow a user to read all of the data, but not allow the same user to create, update or delete any of that data.

Many applications only need all-or-nothing data protection, but some application types need per-row protection. Here are some examples:

  • Protection of personal data, e.g. e-commerce systems which hold personal data for its users.
  • Applications with groups of users that need to be isolated from each other. As a developer I’m sure you know GitHub, which is one (extreme) example of such a system. Generally, these types of systems are referred to as multi-tenant systems.
  • Applications where have hierarchical data needs, e.g. where a Manager-to-staff relationship exists and say staff can access one set of data, but the Manager can see all the data their staff can access. I add this as it covers the implementation of filtering data in different ways depending on the type of user, e.g. a manager or a staff user.

The rest of this article is about applications that need per-row protection and how you might implement that with EF Core.

The two elements of per-row protection

In its core per-row protection needs two things:

  1. A protect key in each row used to define who can access that data.
  2. The user needs to provide a corresponding key than can be compared with protect key in the row.  

The protect key I refer to in the list above about could be a unique value related to an individual user, a group of users, or some form of access key. This protect key is sometimes referred to as the user Id or the tenant Id, and in this article I also use the name “OwnedBy”.

Typically, the “OwnedBy” protect key is held in the information about the currently logged-in user. In ASP.NET Core this is called the ClaimsPrincipal, and it contains a series of Claims (see this article for an introduction on Claims and ClaimsPrincipal). Below is a screenshot of the basic claims of a logged-in user called Staff@g1.com.

As you can see the User has a nameidentifier claim, which a unique string for that user, often referred to as the UserId. In the diagram below I use the UserId as the protect key to select only the addresses in the UsersAddresses table that are link this this user. At the same time the UsersAddresses table has a corresponding protect key held in the “OwnedBy” property/column.

This diagram shows the filter part of the per-row protection system: we get a protect key from the ClaimsPrincipal and use it to filter any read of the protected data. The key will vary, and the filter expression will vary, but the approach will be the same in all cases.

The other part is how we add the protect key to an entity class/database table (what I call “marking an entity”), but that I will explain in the implementation section.

Example 1: Implementation of per-row protection on personal data

I am now going to implement the two elements of the protect key, which are:

  1. Marking data with the per-row protect key.
  2. Filtering data using the user-provided protect key.

There are a number of ways to do this, but I will be placing as much of the per-row protection code inside EF Core’s DbContext. This simplifies your own CRUD/business code, and more importantly it makes sure the per-row protection doesn’t get forgotten in the rush to get your code finished.

What I describe below is the same for all per-row protection, but I use the example of protecting data linked to a user. I go into detail of each step in this process, but in later examples I assume you have read this section for the general approach.

1. Marking data with the per-row protect key

The first step is to “mark” each row with the correct protect key, which I refer to at the “OwnedBy” property, i.e. that row is “owned” by the given key. I want to automatically mark any new protected table rows with the correct protect key deep inside the application’s DbContext. I do this by creating an interface, IOwnedBy, (see below) which I apply to every entity class that needs row-level protection. That interface allows me to detect inside SaveChanges any newly created data that needs the OwnedBy” property filled in before saving to the database.

Let’s look at parts of the code that do this, starting with the IOwnedBy interface.

1a. IOwned interface

public interface IOwnedBy
    //This holds the UserId of the person who created it
    string OwnedBy { get; } 
    //This the method to set it
    void SetOwnedBy(string protectKey);

We also create a class called OwnedByBase that implements that interface and inherits that interface, which makes it easier to add the protection code to any entity classes (i.e. classes that are mapped to the database by EF Core). Here is a link to the PersonalData class in the example code that shows this in action.

1b. Override the SaveChanges and calling MarkCreatedItemAsOwnedBy

The next part is override the SaveChanges/SaveChangesAsync in the application’s DbContext. Here is SaveChanges version (Async version the same, but with async), so I have a common method called MarkCreatedItemAsOwnedBy which sets the “OwnedBy” property.

public override int SaveChanges
    (bool acceptAllChangesOnSuccess)
    return base.SaveChanges(acceptAllChangesOnSuccess);

Note the _userId on line 4. That is the UserId taken from the nameidentifier Claim – I show how you get that in the next section.

The MarkCreatedItemAsOwnedBy extension method uses EF Core’s ChangeTracker to find the newly created entities and then checks if the entity has the IOwnedBy interface. The code looks like this:

public static void MarkCreatedItemAsOwnedBy(
    this DbContext context, string userId)
    foreach (var entityEntry in context.ChangeTracker.Entries()
        .Where(e => e.State == EntityState.Added))
        if (entityEntry.Entity is IOwnedBy entityToMark)

The effect of all that is that the OwnedBy property is filled in with the current user’s UserId on any class that has the IOwnedBy interface.

1c. Getting the UserId into the application’s DbContext

Ok, the first two parts are fairly basic, but getting the UserId into the DbContext is a bit more complex. In ASP.NET Core its done by a series of dependency injection (DI) steps. This can be a bit hard to understand so here is a diagram of how it works.

Here is the method extract the UserId from the current user’s Claims.

public class GetClaimsFromUser : IGetClaimsProvider
    public GetClaimsFromUser(IHttpContextAccessor accessor)
        UserId = accessor.HttpContext?
            .User.Claims.SingleOrDefault(x => 
                x.Type == ClaimTypes.NameIdentifier)?.Value;

    public string UserId { get; private set; }

There are lots of null handling because a) there will be no HttpContext on start, and b) the NameIdentifier Claim won’t be there if no one is logged in. This class is registered in DI in the ConfigureServices method inside the Startup class. That means when the application’s DbContext is created the UserIdFromClaims class is injected.

NOTE: In this case I am injecting the UserId, which is found in the nameidentifier Claim. But you will see later that the same process will allow us to access any Claim in the User’s data.

Here is constructor of the application’s DbContext showing the normal options parameter and the added IGetClaimsProvider data.

public class PersonalDbContext : DbContext
    private readonly string _userId;

    public PersonalDbContext(
        DbContextOptions< PersonalDbContext> options, 
        IGetClaimsProvider userData)
        : base(options)
        _userId = userData.UserId;
//... rest of class left out

The end result of all that is the application’s DbContext has access to the user’s id.

2. Filtering per-row protected data

Now that I have the per-row protected data marked with a protect key (in this case the UserId) I need to apply a filter to only allow users with the correct protect key to see that data. You could write your own code to restrict access to certain data, but with EF Core there is a much better approach – Global Query Filters (shortened to Query Filters in this article).

You can apply Query Filters to any entity class used by EF Core and they pre-filter every read of that entity class. Here is the code that sets this up for the entity class PersonalData in the application’s DbContext.

protected override void OnModelCreating
    (ModelBuilder modelBuilder)
        .HasQueryFilter(x => x.OwnedBy == _userId);

In the UserId example that means a user can only retrieve the PersonalData entity classes that have their UserId, and that filtering is done inside the application’s DbContext so you can’t forget.

NOTE: There is a way to remove any Query Filter from an entity class by adding the IgnoreQueryFilters method to any query. Obviously, you need to be careful where to do this as it removed your security, but it’s pretty clear what you are doing.

Example 2: Implementing a multi-tenant system

The first example was dealing with personal data, which is useful but it’s a simple problem which could have written into your CRUD or business logic code. But when it comes more complex per-row protection problems, like multi-tenant systems, then all that code I just showed you is a much better solution to hand-coding data protection.

As an example of a multi-tenant system I’m going to produce an application to manage the stock in a shop. To cover the cost of the development and hosting I want to sell the application to lots of shops. To do this I need to make sure that each shop’s inventory data is kept separate from other shops. To do this I need to:

  1. When a new shop is signed up I need to create a unique protect key for that shop.
  2. An admin person needs to assign a user to a specific shop.
  3. I need to add the shop’s protect key (called ShopKey) as a claim in the logged-in user’s ClaimPrincipal.
  4. I need to add the per-row protection to all the entity classes/tables that are used by a shop.

Let’s look at each of these stages in turn.

1. Creating a unique protect key for each shop

When a new shop wants to join our inventory control application I need to add a new row to the Shops table. This might be done automatically via some signup system or via an admin person. In my example I use the primary key of the new Shop entity class as the protect key.

2. Assigning a user to a specific shop

Someone (or some service) needs to manage the users that can access a shop. One good approach is to make the person who signed up for the service an admin person, and they can register new users to their shop.

There isn’t anything built in to ASP.NET Core’s authorization system to do this, so we need to add an extra class/table to match UserId’s to the shop key. Here is my MultiTenantUser class that holds the data protection information for each shop user.

public class MultiTenantUser : IShopKey
    public string UserId { get; set; }
    public int ShopKey { get; private set; }

    //other code left out for clarity

This is very simple: I use the UserId provided by ASP.NET Core as the primary key and then set the ShopKey to the admin person’s ShopKey.

3. Lookup the user’s ShopKey and add it as a claim in the logged-in user’s ClaimPrincipal.

In the personal data example I used an existing claim, nameidentifier, in the ClaimsPrincipal as the protect key. In the multi-tenant example the big difference is I need to add a externally held protect key to the ClaimsPrincipal.

NOTE: In my implementation I use cookie-base authentication, but I could have used JWT tokens, social logins etc. The approach I share will work with any of these, but the implementation of how you add a new claim to the ClaimsPrincipal will be different for each authentication type.

In part 1, a better way to handle authorization in ASP.NET Core, I already showed how to add extra Claims to the User – in that article it was a Permissions Claim to help with feature authorization. In this article use the same approach as Part 1 to add the ShopKey to the user’s Claims. That is, I tapped into an event in the Authorization Cookie called ‘OnValidatePrincipal’ (here is a link to the lines in the example application startup class that sets that up). This calls the code below:

public class DataCookieValidate
    private readonly DbContextOptions<MultiTenantDbContext> 

    public DataCookieValidate(DbContextOptions<MultiTenantDbContext>
        _multiTenantOptions = multiTenantOptions;

    public async Task ValidateAsync(CookieValidatePrincipalContext context)
        if (context.Principal.Claims.Any(x => 
            x.Type == GetClaimsFromUser.ShopKeyClaimName))

        //No ShopKey in the claims, so we need to add it. 
        //This is only happens once after the user has logged in
        var claims = new List<Claim>();

        //now we lookup the user to find what shop they are linked to
        using (var multiContext = new MultiTenantDbContext(
              _multiTenantOptions, new DummyClaimsFromUser()))
            var userId = context.Principal.Claims.Single(x => 
                 x.Type == ClaimTypes.NameIdentifier).Value;
            var mTUser = multiContext.MultiTenantUsers
                 .SingleOrDefault(x => x.UserId == userId);
            if (mTUser == null)
                throw new InvalidOperationException($"error…”);

        var identity = new ClaimsIdentity(claims, "Cookie");
        var newPrincipal = new ClaimsPrincipal(identity);
        context.ShouldRenew = true;  

NOTE: In Part1 I give you a detailed description of the stages in the ValidateAsync method, but for authorization. However the steps are very similar so see “How do I turn the Roles into a Permissions Claim?” for a step-by-step breakdown of how the code works.

This code is called for every HTTP request, but the first line quickly returns on all but the first request after a login. On first login the ShopKey claim isn’t in the ClaimsPrincipal so it has to look up the user in the MultiTenantUsers table to add that key to the ClaimsPrincipal. The last line of the code updates the ASP.NET Core authentication cookie content so we don’t have to do this again, which means it’s very quick once it has run once.

4. Add the per-row protection to all the entity classes linked to a shop

This is the same process as I described in the first example of personal data, so I’m going to whiz through the code.

First, I need an interface that every protected shop entity class has to have. In this example it’s called IShopKey, and here is it applied to the StockInfo entity class, with lines 7 to 11 implementing the IShopKey requirements.

public class StockInfo : IShopKey
    public int StockInfoId { get; set; }
    public string Name { get; set; }
    public int NumInStock { get; set; }

    public int ShopKey { get; private set; }
    public void SetShopKey(int shopKey)
        ShopKey = shopKey;


    public Shop AtShop { get; set; }

Then I need to extract the ShopKey claim so I can access it in the Shop application’s DbContext. Here is the variant of the GetClaimsProvider that does that.

public class GetClaimsFromUser : IGetClaimsProvider
    public const string ShopKeyClaimName = "ShopId";
    public int ShopKey { get; private set; }

    public GetClaimsFromUser(IHttpContextAccessor accessor)
        var shopKeyString = accessor.HttpContext?
            .User.Claims.SingleOrDefault(x => 
                  x.Type == ShopKeyClaimName)?.Value;
        if (shopKeyString != null)
            int.TryParse(shopKeyString, out var shopKey);
            ShopKey = shopKey;

In this case I convert the claim value, which is a string, back into an integer as that is more efficient in the database. The query filters in the DbContext’s OnModelCreating method look like this.

protected override void OnModelCreating(ModelBuilder modelBuilder)
        .HasQueryFilter(x => x.ShopKey == ShopKey);
        .HasQueryFilter(x => x.ShopKey == ShopKey);
        .HasQueryFilter(x => x.ShopKey == ShopKey);

NOTE: You need to think hard about the query inside the Query Filters, as they are called on every database read of that emtity. Make sure they are as optimised as possible and add indexes to the protect key column in the database for in each protected entity class.

The DbContext parts can be found at:

Example 3 (advanced): Handling hierarchical accesses

UPDATE: I rushed this section and I wasn’t very happy with it. In the new article Part 4, Building a robust and secure data authorization with EF Core, I build a proper implementation of a hierarchical system – I recommend you go and read that instead.

One of my clients needed multi-tenant system that handled the idea of a group manager, district managers etc. that can get data for reports from a series of shops. This means that the filter rules change depending on the how high the user is in the hierarchy.

I have extended my multi-tenant example such that shops can have district managers. This means:

  • A user linked to a shop, say Dress4U, can only access the information about that shop.
  • But district managers can access the group of shops, say Dress4U, Shirt4U and Tie4U, can see information for all the shops they manage.

This requires us to add four new parts to the existing multi-tenant shop inventory application. They are:

  1. The MultiTenantUser class must change to tell us if the user is a shop worker or a district manager.
  2. When a user logs in I need to see if they are a district manager, and if they are I need to add a DistrictManagerId claim to the ClaimsPrincipal.
  3. The Shop and StockInfo need a reference to an (optional) district manager.
  4. The query filters now need to change depending on whether the current user is a shop-only user or a district manager.

I’m not going to show you the code for all the steps in detail as this article is already very long. What I want to focus on is the Query Filters.

In ASP.NET Core if you ask for an instance of the application’s DbContext, then a new one is created per HTTP request (a scoped lifetime). But, for performance reasons EF Core builds the configuration on first use and caches it. This means you can’t change Query Filters using an If-then-else command – instead you can use the ?: operators in the Query Filter’s LINQ expression.

So, for the multi-tenant application with district manager support the OnModelCreating method looks like this.

 protected override void OnModelCreating(
    ModelBuilder modelBuilder)
    //Altered query filter to handle hierarchical access
        x => DistrictManagerId == null
            ? x.ShopKey == ShopKey
            : x.DistrictManagerId == DistrictManagerId);
        x => DistrictManagerId == null 
            ? x.ShopKey == ShopKey
            : x.DistrictManagerId == DistrictManagerId);

    //… other query filters removed for clarity

Note that the DistrictManagerId is injected into the DbContext using the same system and for the ShopKey. Have a look in the DataLayer for the multi-tenant entity classes and the multi-tenant DbContext.


Another long article, but I have given you a lot of details on how to implement a per-row data protection system. As I said at the beginning feature authorisation (covered in Part1) is much more common than per-row data protection. But if you need per-row data protection then this article shows how you can implement it.

I started with an example of using the user’s unique id to control access to personal data. That also introduced all the parts needed for per-row data protection. I then went onto two more complex per-row protection usages: a multi-tenant system and a hierarchical system. The hierarchical system is complex, but one of my clients needed that (and more!), so it is used.

Security is a very important topic, and one that can be tough to implement with no security loopholes. That is why my implementation places all of the per-row data protection is inside the application’s DbContext. That reduces duplication of code and makes sure per-row data protection is on by default. Personally, I also add unit tests that check I haven’t forgotten to add the query filters and any protection interfaces to the entity classes – you can never be too careful on security.

Happy coding.

PS. Don’t forget to sign up to Jerrie Pelser’s, ASP.NET Weekly newsletter if you are interested in ASP.NET or Entity Framework. It is the most important newsletter I get.

Part 1: A better way to handle authorization in ASP.NET Core

Last Updated: July 31, 2020 | Created: December 14, 2018

I was asked by one of my clients to help build a fairly large web application, and their authentication (i.e. checking who is logging in) and authorization (i.e. what pages/feature the logged in user can access) is very complex. From my experience a knew that using ASP.NET’s Role-based approach wouldn’t cut it, and I found the new ASP.NET Core policy-based approach really clever but it needed me to write lots of (boring) policies.

In the end I created a solution for my client and this article describes the authorization part – I call it Roles-to-Permissions (the name will make more sense as you read the article). I have also build an example ASP.NET Core application, with all new code to support this article. This example application is quite different from my client’s system as I tap into ASP.NET Core built-in Identity system (the client’s system needed OAuth2). The example application contains about 60 lines that I copied (with my client’s permission) from the original implementation to create an open-source version (MIT licence) you can use.

This article is part of a series on authorization in ASP.NET Core

UPDATE: New articles added to this series

There was a lot of interest in this article so I have written some follow-on articles that answer some of the comments/questions from the first two articles. If you want to add the features described in this article use the new new, improved example application in the repo PermissionAccessControl2 and look at the Part 7 article.

NOTE: you can Clone the GitHub repo and run locally – it uses in-memory databases so it will run anywhere. The application was written using ASP.NET Core 2.1.

This is a long article, so here links to the major parts:

TL;DR; – summary

  • ASP.NET Role-based authorization system works for systems with simple authorization rules, but it has limitations, like the fact that you have to republish your code if you change the authorization rules.
  • Roles, with names like “Manager” or “ExternalBuyer” makes sense for users (human or external services) as they define a “Use Case” of what the user should be able to do.
  • But Roles don’t work well when applied to ASP.NET Core actions or Razor Pages. Here you need a much more fine-grained solution, with names like “CanRequestHoliday”, “CanApproveHoliday” – I call these Permissions.
  • The solution is to map the user’s Roles to a group of Permissions and store these in the User’s Claims.
  • Then I use ASP.NET Core’s new policy-based authorization system to check that the User’s Permissions Claims contains the Permission placed on the action/page they want to access.
  • There is an open-source example ASP.NET Core application to go with this article.

Setting the Scene – a look at different application security needs

If you understand ASP.NET’s authorization and authentication features then you can skip this section.

There are billions of web applications and the control of what you can do ranges for “anyone can do anything”, e.g. Google search, up to some military systems where access needs keys, biometrics etc. When you need to prove you are a valid user of the system, say by logging in, that is referred to as authentication. Once you are logged in then what you can do is controlled by what is called authorization.

Authorization breaks down into two parts:

  1. What data can I access? For instance, you can see your personal information, but not other people’s personal information.
  2. What features you can use? For instance, are you allowed to change the title of a book that you can see?

NOTE: This article describes a way to manage the second part, what features can you use.

ASP.NET MVC and now ASP.NET Core have various systems to help with authorization and authentication. Some systems only need a simple authorization – I could imagine a very simple e-commerce system could get away with: a) No logged in – browsing, b) Logged in – buying, and c) Admin – Add/Remove items for sale. This could be done using ASP.NET Role-based authentication.

But many business-to-business (B-to-B) systems have more complex authorization needs. For instance, think of a human resources (HR) system where people can request holiday leave and their manager has to approve those requests – there is lot going on inside to ensure only the correct users can use these features.

Systems like my example HR system often end up with lots of complex authorization rules. My experience is that the ASP.NET Role-based authentication starts to have problems implementing this type of system, which is why I created the Roles-to-Permissions code.

Another type of application that could benefit from the Roles-to-Permissions approach are subscription systems, where the features a user can access depend on what subscription they paid for. The Roles-to-Permissions approach can control the features that as user can access based on the subscription they bought.

Role authorization: what is it and what are its limitations?

Roles authorization has been around for years in the ASP.NET MVC application, and I have used it in a number of applications. Here is an example of a ASP.NET Core controller that can only be accessed by logged in users with either the “Staff” Role or the “Manger” role.

[Authorize(Roles = "Staff,Manager")]
public ActionResult Index()
    return View(MyData);

This works for applications that have fairly simple and well-defined Roles, like User/Admin or Staff/Manager/Admin, then Roles is a good choice. But here are some of the problems I have found:

  1. If you have lots of roles you can end up with long Authorize attributes, e.g.  [Authorize(Roles = “Staff,HrManager,BizManager,DevManage,Admin,SuperAdmin”)].
  2. Because Authorize is an attribute then the string has to be a constant, e.g. you can’t have $”{RoleConstants.StaffRole}”. This means if things change you are editing strings, and you could misspell something really easily.
  3. The big one for me is your authorization rules are hard-coded into your code. So, if you want to change who can access a certain action you have to edit the appropriate Authorize attributes and redeploy your application.

My experience from previous applications using Roles-based authorization is me constantly having to go back and edit the authorize Roles part as I develop or refine the application. I have been looking for a better way for some time, and my client’s requirements spurred me on to find something better than Roles authorization.

The architecture of the Roles-to-Permissions system

1. Introducing Roles, Permissions and Modules

It turns out that there is nothing wrong with the idea of a user having Roles. A user (human or an external service) can typically can be described by their function or department, like “Developer” or “HR”, maybe with side Roles like “HolidayAdmin”. Think of Roles as “Use Cases for users.

NOTE: In the example application I have Roles of “Staff”, “Manager”, and “Admin.

But the problem is that Roles aren’t a good fit for the actions in the Controllers. Each Controller action has a little part to play in a Role, or to turn it around, a Role is made up of a series of Controller actions that the Role allows you to access.  I decided I would call the authorization feature on each action a “Permission”, and I used an Enum to define them. A permission Enum member might be called “CanReadHoliday”, “CanRequestHoliday”, “CanApproveHoliday”, etc.

NOTE: In the example application I have Permissions on my ColorController of “ColorRead”, “ColorCreate”, “ColorUpdate”, and “ColorDelete”.

Now that we have permissions we can provide another feature that of controls access to optional features, e.g. features that a user only has based on their subscription to the service. There are many ways of handling features but by combining optional features into the permissions makes it simpler to setup and control.

NOTE:  In the example application I have Permissions called “Feature1” and “Feature2” which are mapped to Modules with the same name.

2. How this is implemented

Having defined my terms, I’m going to give you an overview of the process. It consists of two parts: the login stage and the normal accesses to the web site. The login stage is the most complex with lots of magic goes on in the background. Its basic job is to convert the user’s Roles into Permissions and add it to the User’s information.

I have set up my example application to store the user’s claims in a cookie which is read in with every HTTP request and turned into a ClaimsPrincipal, which can be accessed in ASP.NET Core by the HttpContext property called “User”.

Here is a diagram of that login stage. It might not make a lot of sense yet, but I describe each part in the rest of the article. This diagram is to give you an overview.

UPDATE: In article “Part 3: A better way to handle ASP.NET Core authorization – six months on” I show a way to handle Roles via the Roles-to-Permissions database. That makes the approach more useful with other authentication approaches such a social media, AzureAd etc.

The second part is simpler and covers what happens every time the logged-in user accesses a protected Controller action. Basically, I have a policy-based authorization with dynamic rules that checks the current User has the permission needed to execute the ASP.NET action/razor page.

NOTE: Don’t forget there is example application if you want to look at the actual code.

Now I’m going to build up the Roles-to-Permissions in stages and explain what each part does.

Why I used Enums for the Permissions

One of the down-sides of using Roles is it used strings, and I’m a little bit dyslexic. That means I can type/spell things incorrectly and not notice. Therefore, I wanted something where intellisence would prompt me and if I still typed it incorrectly it would be a compile error. But it turns out there are a couple of other reasons that make using an Enum for the permissions a good idea. Let me explain.

In a big application there could be hundreds of Permissions. This lead to two problems:

  1. If I use Cookie Authorization there is a maximum size of 4096 bytes for the Cookie. If I had hundreds of long strings I might start to fill up the Cookie, and I want some room for other things like my data authorization. If I can store the Enums permissions as a series of integers it’s going to be much smaller than a series of strings.
  2. Secondly, I want to help the Admin person who needs to build the mapping from Roles to permissions. If they need to scroll through hundreds of permission names it could be hard to work out which ones are needed. It turns out Enum members can have attributes, so I can add extra information to help the Admin person.

So, here is part of my Permissions Enum code

public enum Permissions
    //Here is an example of very detailed control over something
    [Display(GroupName = "Color", Name = "Read", Description = "Can read colors")]
    ColorRead = 0x10,
    [Display(GroupName = "Color", Name = "Create", Description = "Can create a color entry")]
    ColorCreate = 0x11,
    [Display(GroupName = "Color", Name = "Update", Description = "Can update a color entry")]
    ColorUpdate = 0x12,
    [Display(GroupName = "Color", Name = "Delete", Description = "Can delete a color entry")]
    ColorDelete = 0x13,

    [Display(GroupName = "UserAdmin", Name = "Read users", Description = "Can list User")]
    UserRead = 0x20,
    //This is an example of grouping multiple actions under one permission
    [Display(GroupName = "UserAdmin", Name = "Alter user", Description = "Can do anything to the User")]
    UserChange = 0x21,

    [Display(GroupName = "Old", Name = "Not used", Description = "example of old permission"
    OldPermissionNotUsed = 0x40,
//... other code left out

The things to note are:

  • I show two types of permissions.
    • First four (lines 4 to 11) are fine-grained permissions, almost one per action.
    • Next two (lines 13 to 17) are more generic, e.g. I have a specific “UserRead”, but then one permission called “UserChange” which allows create, update, delete, lock, change password etc.
  • Line 5, 7, etc. Notice that I give each enum a specific number. If you are operating a 24/7 application with a new version seamlessly replacing the old version, then the Permission numbed mustn’t change otherwise user’s Claims be wrong. That is why I give each enum a specific number.
  • Line 19. I also support the Obsolete attribute, which stops the Permission appearing in the listing. There are plenty of scary stories about reusing a number with unintended consequences. (Also, it you try to use something marked as Obsolete you get a warning).
  • Line 4 etc. I add a Display Attribute to each Permission Enum. This has useful information that I can show lots of useful information to help the person who is building a Role.
  • Line 4, 6, 8, 10. I “Group” permissions that are used in the same place. This makes it easier for the Admin person to find the things they want. I also number in Hex, which gives me 16 possible permissions in a Group (I tried 10 and you could go over that, so 16 is better).

Here is a list of some of the Permissions in my example application listed via the Users->List Permissions nav dropdown.

And the code that produced that output (link to PermissionDisplay class for the whole thing)

public static List<PermissionDisplay> GetPermissionsToDisplay(Type enumType) 
    var result = new List<PermissionDisplay>();
    foreach (var permissionName in Enum.GetNames(enumType))
        var member = enumType.GetMember(permissionName);
        //This allows you to obsolete a permission and it won't be shown as a
        //possible option, but is still there so you won't reuse the number
        var obsoleteAttribute = member[0].GetCustomAttribute<ObsoleteAttribute>();
        if (obsoleteAttribute != null)
        //If there is no DisplayAttribute then the Enum is not used
        var displayAttribute = member[0].GetCustomAttribute<DisplayAttribute>();
        if (displayAttribute == null)

        //Gets the optional PaidForModule that a permission can be linked to
        var moduleAttribute = member[0].GetCustomAttribute<PermissionLinkedToModuleAttribute>();

        var permission = (Permissions)Enum.Parse(enumType, permissionName, false);

        result.Add(new PermissionDisplay(displayAttribute.GroupName, displayAttribute.Name, 
                displayAttribute.Description, permission, moduleAttribute?.PaidForModule.ToString()));

    return result;

How to handle optional/paid-for features?

My client provides a Business-to-Business application and plans to add new features that customers can subscribe to. One way to handle this would be create different Roles, like “Manager”, “ManagerWithFeature1”, “ManagerWithFeature2” or add separate Feature Roles that you have to manually apply to a user. That works but is pretty horrible to manage, and human error could cause problems. My preferred system is mark Permissions linked to a paid-for feature filter them based on the User’s subscriptions.

Marking Permissions as linked to a module is easy to do with the Enums – I just add another attribute. Here an example of Permissions linked to a Module (see line 5).

public enum Permissions
    //… other Permissions removed for clarity

    [Display(GroupName = "Features", Name = "Feature1", Description = "Can access feature1")]
    Feature1Access = 0x30,
    [Display(GroupName = "Features", Name = "Feature2", Description = "Can access feature2")]
    Feature2Access = 0x31

The paid-for modules are again represented by an Enum, but one marked the [Flags] attribute because a user can have multiple modules that they have subscribed to. Here is my PaidForModules Enum code

public enum PaidForModules : long
    None = 0,
    Feature1 = 1,
    Feature2 = 2,
    Feature3 = 4

NOTE I add “: long” to the Enum which gives me up to 64 different modules in my system.

What happens is that Permissions linked to a Module that the user hasn’t subscribe to are filtered out when working out what permissions the user should have – I show how later. This makes the setting up the Roles much simpler, as you build each Role with all the Permissions that make sense for that role, including features mapped to a paid-for module. Then, at login time, the system will remove any Permissions the current user doesn’t have access to. That is simpler for the Admin person and more secure for the application.

How do I turn the Roles into a Permissions Claim?

I my client’s system we uses 0Auth2 authentication, but for this example I used ASP.NET Core IdentityRole to hold the Roles that a user has. That means I can use all of the ASP.NET Core built-in Identity code to set up the Users and Roles. But how do I convert the User’s Roles to a Permissions Claim?

UPDATE: Two improvements added in later articles:

  1. In article called “Part 3: A better way to handle ASP.NET Core authorization – six months on” I show an easier way to turn Roles into permission claims using the UserClaimsPrincipalFactory- see this section.
  2. In the second version of the code in the PermissionAccessControl2 repo I improved/simplified the code called by ‘OnValidatePrincipal’ and provided versions for various extra features I added. See the folder “AuthorizeSetup” for the code.

Again there are few ways to do it, but in the end I tapped into an event in the Authorization Cookie called ‘OnValidatePrincipal’ (here is a link to the lines in the example application startup class). This calls the code below, but be warned it’s pretty complex so here is a summary of the steps it goes through:

  1. If the Claims already have the Permissions claim type then nothing to do so return quickly.
  2. Then we get the Roles the user has from the Role Claim
  3. I need to access my part of the database. I can’t use dependency injection, so I use the extraAuthDbContextOptions, which is a singleton that I can provide at startup.
  4. Then I get all the permissions for all of the roles, with a Distinct to remove unnecessary duplicates.
  5. Then I filter out any permissions that are linked to a Module that the user doesn’t have access to.
  6. Then I add a permissions Claim containing all the Permissions the user is allowed, but packed as hex numbers in a single string so that it doesn’t take up so much room (I used Hex format as it made debugging easier).
  7. Finally I have to create a new ClaimsPrincipal and tell ASP.NET Core to replace the current ClaimsPrincipal, plus set the all-important context.ShouldRenew  to true, which updates the Cookie, otherwise this complex (slow) method on every HTTP request!
public async Task ValidateAsync(CookieValidatePrincipalContext context)
    if (context.Principal.Claims.Any(x => 
        x.Type == PermissionConstants.PackedPermissionClaimType))

    //No permissions in the claims so we need to add it
    //This is only happens once after the user has logged in
    var claims = new List<Claim>();
    foreach (var claim in context.Principal.Claims)

    var usersRoles = context.Principal.Claims
        .Where(x => x.Type == ClaimTypes.Role)
        .Select(x => x.Value)
    //I can't inject the DbContext here because that is dynamic, 
    //but I can pass in the database options because that is a 
    //From that I can create a valid dbContext to access the database
    using (var dbContext = new ExtraAuthorizeDbContext(_extraAuthDbContextOptions))
        //This gets all the permissions, with a distinct to remove duplicates
        var permissionsForUser = await dbContext.RolesToPermissions
            .Where(x => usersRoles.Contains(x.RoleName))
            .SelectMany(x => x.PermissionsInRole)

        //we get the modules this user is allows to see
        var userModules =
                     .SingleOrDefault(x => x.Type == ClaimTypes.Name).Value)
                ?.AllowedPaidForModules ?? PaidForModules.None;
        //Now we remove permissions that are linked to modules that the user has no access to
        var filteredPermissions =
            from permission in permissionsForUser
            let moduleAttr = typeof(Permissions).GetMember(permission.ToString())[0]
            where moduleAttr == null || userModules.HasFlag(moduleAttr.PaidForModule)
            select permission;

          //Now add it to the claim
          claims.Add(new Claim(PermissionConstants.PackedPermissionClaimType,
              filteredPermissions.PackPermissionsIntoString()));    }

    var identity = new ClaimsIdentity(claims, "Cookie");
    var newPrincipal = new ClaimsPrincipal(identity);

    context.ShouldRenew = true;

How do I convert my Permissions into Policy-based authorization?

OK, I now have access to the Permissions via the User’ Claims, but how do I get this turned into something that ASP.NET Core can use for authorization. This is where a .NET developer and friend, Jerrie Pelser helped me.

When I started this project, I emailed Jerrie Pelser, who runs the ASP.NET Weekly newsletter (great newsletter! Do sign up) as I know Jerrie is an expert in authentication & authorization.  He pointed me at a few architectural things and I also found his own article “Creating Authorization Policies dynamically with ASP.NET Core” really helpful.  Jerris’s article showed me how to build policies dynamically, which is exactly what I need.

I’m not going to repeat Jerrie article here (use the link above), but I will show you my PermissionHandler that is used inside the policy to check that the current User’s Permissions claim exists and contains the Permission on the action/Razor Page. It uses an extension method called ThisPermissionIsAllowed which does the check.

public class PermissionHandler : 
    protected override Task HandleRequirementAsync(
        AuthorizationHandlerContext context, 
        PermissionRequirement requirement)
        var permissionsClaim = context.User.Claims
            .SingleOrDefault(c => 
                 c.Type == PermissionConstants.PackedPermissionClaimType);
        // If user does not have the scope claim, get out of here
        if (permissionsClaim == null)
            return Task.CompletedTask;

        if (permissionsClaim.Value

        return Task.CompletedTask;

There are two other classes that are involved in making dynamic policy-based authorisation work. Here are the links to them:

Policies are defined by strings, but as I said I hate strings as I can make a mistake. I therefore created this very simple HasPermission attribute which allows me to apply an Authorize attribute, but using a Permissions Enum

    | AttributeTargets.Class, Inherited = false)]
public class HasPermissionAttribute : AuthorizeAttribute
    public HasPermissionAttribute(Permissions permission) 
       : base(permission.ToString()) { }

That’s pretty simple, but it means I get intellisence when I am adding the Permission.

Putting it all together

So, we have the Permissions in the code and we can apply them using our HasPermissionAttribute to each action we want to protect via authorization. Here is one action taken from the ColorController in my example application.

public ActionResult Index()
    return View(MyData);

We also need to add two tables to whatever database your application uses. The two EF Core entity classes are:

Once the application is up and running an Admin-type user has to:

  1. Create some roles, e.g. “Staff”, “Manager”, etc. using ASP.NET Core Identity code.
  2. Create matching RoleToPermissions for each of the roles, specifying what Permissions map to each Role.

Then, for every new user an Admin person (or some automatic subscription code) has to:

  • Create the user (if an invite-only type application)
  • Add the correct Roles and ModulesForUser for that new user.

Once that is done all the code I have shown you takes over. The user logs in, gets the Permissions and what they can access is managed by ASP.NET Core’s policy-based authentication.

Things I didn’t cover elsewhere

There are a few things I didn’t cover in detail, but here are links to the items:

  • Startup. The important parts about registering things are shown in highlighted lines in this link to the Startup class.  (NOTE: I built the application with ASP.NET Core 2.1, but I know the Identity parts are changing in 2.2, so you might have to update the code I put in the Startup class for newer versions of ASP.NET Core).
  • You don’t need to use ASP.NET Core Identity system at all – I said the client’s version uses an external authentication system. You just have to create a Roles-to-User table so you can assign Roles to each user.
  • I didn’t cover how I Packed/Unpacked the Permissions. You can find the Extension methods for doing that in PermissionPackers.
  • You might want to check for a permission in your razor pages to show/hide links. I created a simple method in the PermissionsExtension class, and used it in the _Layout.cshtml Razor page.


Well that is a long article so well done by getting to the end. I have described an authentication I have built that handles complex authentication rules while being (relatively) easy to understand and manage via the Admin staff. Sure, if you have hundreds of Permissions it’s not to be hard setting up the initial RolesToPermissions, but the Admin has a lot of information to help them.

For me the Roles-to-Permissions approach solves a lot of problems I had in older systems I built using ASP.NET MVC Roles. I have to write some more code, but it makes it a) easier to change the authorization rules and b) helps the Admin person manage applications with lots of Roles/Permissions. I hope it helps you and makes you think of better ways of building better authentication systems for your projects.

UPDATE: See new code in PermissionAccessControl2 repo and new articles.

The new code and articles add more features, and more especially make the code easier to copy into your own application. See article called “Adding the “better ASP.NET Core authorization” code into your app” which gives you a step by step tutorial on how to add the features in this and later articles into your own code.

Further reading

Happy coding. PS. Don’t forget to sign up to Jerrie Pelser’s, ASP.NET Weekly newsletter if you are interested in ASP.NET or Entity Framework. It is the most important newsletter I get

Building high performance database queries using Entity Framework Core and AutoMapper

Last Updated: July 31, 2020 | Created: November 30, 2018

When you are writing Entity Framework Core (EF Core) queries (i.e. reading data from the database) you want them to be quick to run (high performance) and quick to write. It turns out that it’s possible to do that in EF, but the “quick to run” part isn’t that obvious. I make a living around writing EF Core code quickly, and that performs well (I wrote the book, “Entity Framework Core in Action”) and my approach is to use the LINQ Select method (quick to run) and the AutoMapper library (quick to write).

This article is about both “quick to run” and “quick to write”, but majors on AutoMapper, as the Select method is easy to understand, but AutoMapper does take a bit of getting used to. I also mention my EfCore.GenericServices library that uses AutoMapper, which speeds up the writing of database accesses even more.

TR;DR; – summary

  • LINQ Select method normally produces the fastest performing query because they only load what you need, and some calculations can be run in the database (see Posts.Count later) .
  • Writing Select queries contains more lines of code than other query approaches because you have to select each property you want to load.
  • AutoMapper’s ProjectTo method can build the Select query for you, but you need to configure the mapping.
  • Setting up the AutoMapper configuration isn’t total obvious (to me at least) so I give you some examples.
  • My GenericServices library uses AutoMapper and does some of that configuring for you.

This article is aimed at developers who use Microsoft’s Entity Framework library, so I assume you are familiar with C# code and either Entity Framework 6 (EF6.x) or Entity Framework Core library. I don’t assume you know the AutoMapper library, but I don’t detail all the features of AutoMapper and give links to its documentation. The examples in this article use EF Core, but all of the ideas are transferable to EF6.x.

Setting the scene – A quick intro to Entity Framework Core

Database accesses are normally referred to as CRUD operation (Create, Read, Update and Delete), with the read referred to in EF as a query. Queries are normally the most common operation, and often the one you want to be quick (Google and Amazon searches have conditioned us to expect very fast searches). Sometimes the query is a list of things, like products you can buy or trains you could catch, and sometimes you need a single item, like a calendar event you want to change. When the data is stored in a relational database, like SQL Server, a query might need data from multiple parts of the database, called tables, to form the exact data the user wants to see.

The EF Core library makes it easy to access a database by mapping the database tables to .NET classes (known as entity classes), and uses the database pointers (known as foreign keys) between tables to create references (known as navigational properties) to other entity classes. It also turns your LINQ commands into the correct language/format used by the database you want to access. The figure below shows four entity classes, which represent a set of Posts, each with a Blogger who wrote it, and some Tags (via a many-to-many PostTag linking entity class).  The relational properties are represented in red.

In general, the LINQ Select operator in Entity Framework Core (EF Core) can produce database queries that are often perform better than any other approach. This is because the Select method allows you to pick exactly the properties you want, including looking into related classes. This performance gain is especially good when part of the query can be done inside the database, e.g. like counting how many posts a blogger has written (I show that later).

The problem is that writing a Select query, with an assignment for each property, is very repetitious and a bit boring (and boring is bad, as I can make mistakes when I am bored). In this article I show how you can use a library called AutoMapper to automatically build Select queries, which saves you time (and you don’t need to be bored anymore). I also cover one of my libraries that uses AutoMapper to make Create, Read, Update and Delete (known as CRUD) database accesses easier.

In the next section I am going to build some queries to show this database in different ways.

Part 1. Building a simple database query – a list Bloggers

The first display I want to build a list of each blogger, with the number of posts they have written. Here is the type of display I want to show.

This must pick out information from two of entity classes. Now, let’s see the different ways of obtaining that data.

1a. Using Eager loading to load the Post classes

One way to do that would be to use Eager Loading. The code would look like this.

var list = context.Bloggers
    .Include(x => x.Posts)

That gets the data we need, but it pulls in ALL of the post’s content, when I only want to count them. That is very inefficient, because the Posts can be very big. If you need good performance then you don’t want to write your queries this way.

NOTE: You get EVEN WORSE performance if you load the Posts via Lazy Loading, e.g. having a virtual public ICollection<Post> of Posts. That will cause one extra trip to the database for every entry. I get paid to performance tune EF systems and the first thing I look for is Lazy Loading – it’s a major performance drag.

See the Summary of the performance table for detailed timings.

1b. Using a hand-coded Select query

Over the years I have found the LINQ Select queries to be more efficient, where I only select the properties I need into a class I call a Data Transfer Object (DTO), also known as a ViewModel in ASP.NET. Here is my ListBloggersDto class that I am going to copy the data into

public class ListBloggersDto 
    public string Name { get; set; }
    public string EmailAddress { get; set; }
    public int PostsCount { get; set; }

And here is a hand-coded Select query that will only read in the data I actually need.

var list = context.Bloggers.Select(x =>
    new ListBloggersDto
    Name = x.Name,
    EmailAddress = x.EmailAddress,
    PostsCount = x.Posts.Count

This is more efficient as it only loads the data it needs. This is especially true for the count of the Blogger’s Posts – this is done in the database, which stops us loading all the Posts just to count them. The SQL that EF Core would create from this query looks like this (don’t worry if you don’t know SQL – take it from me that is very efficient).

SELECT "x"."Name", "x"."EmailAddress", (
    FROM "Posts" AS "p"
    WHERE "x"."BloggerId" = "p"."BloggerId"
) AS "PostsCount"
FROM "Bloggers" AS "x"

Summary of the performance

In section 12.5.1 on my book “Entity Framework Core in Action” I built a query to load the data to create the book display list (see http://efcoreinaction.com/ for live example site) using three approaches. Here is the result of the three different ways of loading the data. As you can see the Select query was the fastest.

NOTE: I didn’t test Lazy loading as EF Core 2.1 wasn’t released when I was writing that chapter, but I can tell you it would be equal (or maybe worse) to the explicit loading timing.

Now, for our much simpler example the Select method would have a sub-millisecond timing because the query is so simple, and would be much quicker than any of the other loading approaches. But the table above gives you a better comparative of a complex query (see my article “Entity Framework Core performance tuning – a worked example” for more on performance)

Quick to write – AutoMapper

Having looked at the “quick to run” part lets move onto the “quick to write” part. The query examples so far are pretty easy to write, but in bigger Select queries you would have to write many assignments, and that gets tedious. This is where AutoMapper comes in, so lets now recreate the same query using AutoMapper.

1c. Using AutoMapper to build the Select query automatically

AutoMapper is what is known as an object-to-object mapper, and can selectively map from one class to another, handling any relationships. AutoMapper can also produce LINQ code via its ProjectTo<T> method (see Queryable Extensions). This allows you to let AutoMapper build your Select queries for you by matching up names. There are three stages to building a Select query using AuthoMapper:

  1. Create your DTO
  2. Add an AutoMapper configuration,
  3. Then use that AutoMapper configuration in your query.

Let’s go through each part in turn

a) Create my DTO

I need to create my DTO, which is the same as the hand-coded one.

public class ListBloggersDto
    public string Name { get; set; }
    public string EmailAddress { get; set; }
    public int PostsCount { get; set; }

The first two properties have the same name and type as properties in the Blogger class, so they are copied. The interesting property is the PostsCount – this is translated as Posts.Count(). That’s a AutoMapper feature which is helpful.

NOTE: AutoMapper can also ‘flatten’ relationships’: what that means it can pick out a one-to-one relationship by joining together the two names without the dot in the middle. You will see this in part 2, where I list each Post, but access the Blogger’s Name by using the name of the relationship, Blogger, followed by the property’s name, Name, i.e. BloggerName.

b) Configure the AutoMapper mapping via dependency injection

There are lots of ways to configure AutoMapper mappings. For NET Core the recommends using AutoMapper’s Profile and assembly scanning with NET Core’s Dependency Injection (DI).

For this example, I use a DI approach to configuring your AutoMapper mapping in an ASP.NET Core web application. First you add a class that inherits AutoMapper’s Profile class, most likely near to where you define your DTOs. In class’s constructor you add one or more CreateMap methods to set up the mapping. Here is a simple example.

public class BloggerDtosProfile : AutoMapper.Profile
    public BloggerDtosProfile()
        CreateMap<Blogger, ListBloggersDto>();
        // Add other CreateMap’s for any other configs

Then you need to call AutoMapper’s AddAutoMapper() method. In ASP.NET Core that would be in the ConfigureServices method in the Startup class, e.g.

public IServiceProvider ConfigureServices(
    IServiceCollection services)
    // … other setups removed for clarity

The AddAutoMapper() method scans all the assemblies looking for all the classes that inherit AutoMapper.Profile class. It then produces a IMapper instance containing the all the Mappings found.

NOTE: I find the configuring AutoMapper is the hard part, as it relies on me remembering to set up the AutoMapper mapping. I have tried a number of ways to make this better and I think in my EfCore.GenericServices I have got it right. In that I write my DTO and then you need to add an empty interface called ILinkToEntity<TEntity>, where TEntity is your class that EF Core maps to the database. Then the setup of EfCore.GenericServices a) finds the DTOs and then uses the TEntity part of the ILinkToEntity<TEntity> interface to, among other things, form the correct AutoMapper mappings.

c) Use AutoMapper in your query

I’m now going to show you a ASP.NET Core controller so that you can see how the DI works, but you can use AutoMapper in any application you like.

public class HomeController : Controller
    private readonly BlogContext _context;
    private readonly IMapper _mapper;

    public HomeController(BlogContext context, IMapper mapper)   
        _context = context;  
        _mapper = mapper;                      

    public IActionResult Index()            
        var dtos = _context.Bloggers

        return View(dtos);         

I have injected by EF Core content and the AutoMapper’s IMapper instance into the controller via the constructor. Then any of my methods in the ASP.NET Core controller. And the SQL code produced is the same as the hand-coded Select earlier in 1b.

For this small DTO then it isn’t worth using AutoMapper, but most applications have hundreds of mappings like this, and classes that contains a lot more properties. This is where AutoMapper comes into its own.

NOTE: If you are generally doing CRUD operations then you should look at my article “GenericServices: A library to provide CRUD front-end services from a EF Core database” that uses AutoMapper and EF Core to make CRUD very simple to do.

AutoMapper is clever and can work out some relationships for you, but for more complex relationships you need to do some more configuring, which I cover in the next part.

Part 2: Handling more complex database queries – list of posts

The next example is a summery list of all the articles, showing the bogger (author), the post title and the tags the post has.

This is a bit more complex, especially around getting the tag names.

2a. List posts using hand-coded select query

Let’s start with the hand-coded LINQ Select query. First, I need to show you the DTO I use to hold the resulting query.

public class ListPostsDto 
    public string BloggerName { get; set; }
    public string Title { get; set; }
    public DateTime LastUpdated { get; set; }
    public List<string> TagNames { get; set; }

Now the hand-coded Select query.

var dtos = context.Posts.Select(x => 
    new ListPostsDto
    BloggerName = x.Blogger.Name,
    Title = x.Title,
    LastUpdated = x.LastUpdated,
    TagNames = x.TagLinks.Select(y => y.Tag.Name).ToList()

Notice the following:

  • Line 4: I can select the Name of the Blogger by using the navigational property. EF can handle any depth of selection, say the Bogger class had a property called Address, which has its own table holding address information, then I could access the Blogger’s Country via x.Blogger.Address.Country. EF produces efficient SQL (an INNER JOIN) to extract the data from the related tables.
  • Line 7: I want a list of all the Tag Name’s associated with this post. I therefore need to use the PostTag linking table to access the Tag names.

The produces fairly efficient (see note below) SQL query by only extracting the data you need.

NOTE: Handling “many” relationships, i.e. ones that return multiple results like the TagNames, is an area where EF Core can have performance issues. If I left off the .ToList() on the TagNames then it would query the database for each ListPostsDt0 (this is known as the N+1 database queries issue). But in EF Core version 2.1, adding the .ToList() turns the load of all the TagNames into one final database query. This is OK, but can still have problems – see my article “Entity Framework Core performance tuning – a worked example” to see an even better way to handle comma separated strings.

2b. List of posts using AutoMapper and special configuration

AutoMapper is clever enough to automatically map the first three properties in the ListPostsDto – in particular it maps the BloggerName property to Blogger.Name. What it can’t do is work out the mapping for the TagNames, so we need to add some configuration the CreateMap method. I use the original ListPostsDto I used in the hand-coded version, but add the AutoMapper configuration code:

public class ListPostsDto
    public string BloggerName { get; set; }
    public string Title { get; set; }
    public DateTime LastUpdated { get; set; }
    public List<string> TagNames { get; set; }

Now the AutoMapper configuration code.

public class PostsDtosProfile : AutoMapper.Profile
    public PostsDtosProfile()
        CreateMap<Post, ListPostsDto>()
                p => p.TagNames, 
                opt => opt.MapFrom(x => 
                    x.TagLinks.Select(y => y.Tag.Name).ToList()));

The ForMember method (lines 6 to 9) takes two parts: the first part defines the property in the DTO that you are mapping (TagNames, on line 7), and the second part (lines 8 to 9) tells AutoMapper how to generate the data to go into the TagNames property from the input class, which is the Post class.

NOTE: AutoMapper automatically turns the BloggerName into Blogger.Name – this is the “flattening” I referred to earlier. This is a very useful feature and makes handling references to a single relationship really easy.

Let’s see the list articles query using AutoMapper’s ProjectTo method. In case I’ll show you a unit test as you might need this in your system. I also show how to create the IMapper variable using a helper method I created, as that is useful too (and checks you created the Profile class correctly).

public void TestProjectionMappingPosts()
    var config = AutoMapperHelpers

    var input = EfTestData.CreateBloggersWithPosts()
    var list = input.ProjectTo<ListPostsDto>().ToList();

    list.First().BloggerName.ShouldEqual("Ada Lovelace");

Where my helper method CreateMapperConfig, looks like this.

NOTE: Technical point about unit testing. ProjectTo<T> need MapperConfiguration when testing, while Map<T> needs IMapper. Have a look at my unit tests of AutoMapper here for a good example of how to write tests that include AutoMapper.

public static MapperConfiguration CreateMapperConfig<T>() 
    where T : Profile, new()
    var config = new MapperConfiguration(cfg =>
        cfg.AddProfile(new T());
    return config ;

This produces the same SQL as the hand-codes example.

NOTE: In EfCore.GenericServices you can configure the AutoMapper mappings by adding a class which inherits from PerDtoConfig<TDto, TEntity>. When EfCore.GenericServices is scanning for DTOs it also looks for matching PerDtoConfig<TDto, TEntity> classes uses that data to alter the AutoMapper’s MapperConfiguration.


Firstly, I showed you how to create high performance queries by using the LINQ Select method. The Select query isn’t obvious – in fact the EF Core documentation on loading relationships doesn’t even list it even though you can load relationships in a Select method. But the ASP.NET Core EF tutorial lists Select as an option, as I was asked to help develop that tutorial 😊. If you want fast and effective queries then think about using the LINQ Select method.

The problem with the Select method is it takes some work to write it, e.g. when Selects have lots of properties in them then they get long and repetitious (i.e. boring). I showed that AutoMapper can help by automatically build the Select query for you. While AutoMapper is a great tool, but it takes a bit of getting used to, especially around the configuration. But when you have lots of queries then it’s worth the effort.

Along the way I talk about my EfCore.GenericServices library, which uses AutoMapper inside. This handles the configuration of AutoMapper for you, which makes it a bit easier. But on the other hand, you need to learn how to set up EfCore.GenericServices. I have written an introductory article, some documentation, and an example application in the repo that you can run locally (it uses an in-memory database, so it runs anywhere), so don’t be afraid to try it.

Happy coding.

Three approaches to Domain-Driven Design with Entity Framework Core

Last Updated: July 31, 2020 | Created: September 24, 2018

On my article “Creating Domain-Driven Design entity classes with Entity Framework Core@ardalis commented that “your entities all are tightly coupled to EF Core. That’s not good…”. Then I did a podcast with Bryan Hogan where we discussed Domain-Driven Design (DDD) and he goes further than my CRUD-only (Create, Read, Update, and Delete) approach – he says that the entity classes is the perfect place for business logic too.

NOTE: My discussion with Bryan Hogan is now out. You can find the PodCast here.

With such diverging views on the best way to implement DDD in Entity Framework Core (EF Core) I decided to write an article that a) compares normal approach with a DDD approach, and b) compare three different ways to implement DDD in EF Core. This is a detailed look at the issues, hence it is very long! But hopefully useful to those looking to use DDD, or developers that want to consider all the options available to them.

TL;DR; – summary

NOTE: DDD is a massive topic, with many facets. In this article I only look at the entity class issues, which is a tiny part of what DDD is about. I really recommend Eric Evan’s book Domain-Driven Design for a full coverage.

The DDD approach to writing entity classes in EF Core makes every property read-only. The only way to create or update entity data is constructors (ctors), factories or methods in the entity class. This article introduces the DDD entity style by comparing the standard, non-DDD approach, against a basic DDD-styled entity class. This gives us a clear starting point from which I can go on to compare and contrast three difference DDD approaches:

  1. A DDD-styled entity, but not including any references to EF Core commands.
  2. A DDD-styled entity which has access to EF Core’s DbContext and implements CRUD methods.
  3. A DDD-styled entity which has access to EF Core’s DbContext and contains all code that interacts with the entity, i.e. CRUD and more complex business logic.

Comparing a standard entity class with a DDD approach

Let’s start by comparing an implementing something using a standard entity class and a DDD-styled entity class. The aim of this section is to describe the major differences and sets the scene for a look at the subtler differences between the three DDD approaches.

The diagram below shows a standard entity class (i.e. a class that EF Core maps to the database) with read-write access.

And this diagram shows the same entity class but using a DDD approach.

The big difference is that the standard entity class can be created/changed by any external code, but in the DDD-styled entity class you can only create/change data via specific constructors/methods. But why is this a good idea? – here is a list of benefits:

  • External code now has clearly named methods/static factories to call. This makes it much clearer to developers what the entity class supports in terms of creating/changing that class.
  • The code to create/change the class is contained in the class itself, which keeps the code co-located with the data. This makes writing/refactoring of the class much simpler.
  • It stops duplication of code, and in multi-person projects it stops different developers (or even the same developer!) applying different business rules to the same feature.

DDD is about making the domain (i.e. business) rules the focus of the code, so having methods like ChangeDeliveryDate or MarkOrderAsDispatched in your entity class encapsulates business rules with a meaningful name. There is much less possibility of getting things wrong this way.

Pros and Cons of Standard & DDD approaches

Here is summary of the Pros/Cons (advantages/disadvantages) of the two approaches

Approach Pros Cons
Standard Simple.

Minimum code.


Big systems can become quite hard to understand.

Possibly of duplicate code happening.

DDD-styled More obvious, i.e. meaningful named methods to call.

Better control of access to data.

Puts the code next to the data.

Slightly more code to write.


Different views on building DDD-styled entity classes

Having introduced the DDD approach I now want to look at three different approaches to implementing DDD entity classes. Here is a diagram to show you the three DDD approaches, showing what code the entity classes contain. I use two terms that I need to define:

  • Repository: A repository pattern provides a set of methods to access the database. These method ‘hide’ the code needed to implement the various database features you need.
  • C(r)UD: This is Create, (Read), Update, and Delete – I use the term C(r)UD to separate the Read from the functions that change the database (Create, Update are Delete).
  • Query Objects: This is a design pattern for building efficient database queries for EF. See this article from Jimmy Bogard on this topic.
  • Business logic (shortened to Biz Logic): This is more complex processes that go beyond simple validation. They may have complex calculations (e.g. pricing engine) or require access to external systems (e.g. sending an email to a customer).

As you will see in the diagram below, the further to the right you go the more external code is moved inside the entity classes.

NOTE: I will use the terms POCO-only class, C(r)UD only and C(r)UD & Business Logic when referring to these three approaches.

1. The different ways of reading data

The first big difference is how the data is read from the database. When using a repository pattern, you tend to read and as well as write via the repository pattern. I personally have found (some) repository patterns can lead to poor performing code, mainly because it’s hard to build a fully featured repository with the correct adapters for the front-end. I therefore use Query Objects, normally defined in the services layer so that they can adapt the data to the needs of the front end.

I’m not going to cover this here as I have an article called “Is the repository pattern useful with Entity Framework Core?” which goes into it in detail, with this section on query objects.

2. The different ways of updating relationships

The big difference between the POCO-only class approach and the other DDD versions is that the POCO-only version doesn’t have any access to the DbContext, so it can’t load relationships. To show why this matter, let’s introduce a Book entity class a collection of Reviews, e.g. (5 stars – “It’s a great book” says Jill). Now we want to implement a method to allow a user to add a new Review entity class to an existing Book entity instance. The steps are:

  1. Get the new review information, with the Book’s primary key.
  2. Load the Book entity instance using the primary key.
  3. Call a method in the loaded Book instance to create a new Review and add it to the Book’s Reviews collection.

The problem comes if the Reviews collection hasn’t been loaded when the Book entity instance was loaded. If we assume the Reviews collection was loaded and its wasn’t, then adding a new review will either a) fail with a null collection or b) silently delete all the other reviews! Therefore, something has to make sure the collection is loaded. Here are some possible implementations for the POCO-only version, and the other two DDD versions:

1.a Handling adding a Review to POCO-only version

In a POCO-only entity class there is no references to EF Core. This means you need something else to handle the correct loading of the Book with its Review collection. The standard way to do this is via a repository pattern. The repository would contain an AddReview method that would load the Book entity instance, along with loading the Reviews collection before it calls the Book’s AddReview method to update the Books _reviews backing field. Here is some example code:

1.b Handling adding a Review within the DDD-styled entity class

The limitation of the POCO-only entity class is you need to rely on the caller to pre-load the Reviews collection. Because the POCO-only entity class can’t access any of EF Core’s classes or methods. In the other two types you can access EF Core, so you can move loading of the Reviews collection inside the entity class. Here is the code:

NOTE: When using the C(r)UD only approach the code in the ASP.NET controller has a repetitive pattern:  load the entity and call a named method, which makes it a candidate for building library to handle this. This is exactly what I have done with the EfCore.GenericServices library, which can work with normal and DDD-styled entity classes. This the article “GenericServices: A library to provide CRUD front-end services from a EF Core database” for more on this.

Comparing the two approaches to adding a review

The differences are subtle, but important. In the POCO-only class the update relies on an external piece of code, in this case a repository pattern, to correctly update the reviews. In the C(r)UD only case the DDD access method, AddReview, handles the whole process.

Now, if you are using a repository pattern everywhere then this difference is small. However, it does allow a small loophole in which someone could bypass the repository and calls AddReview directly, which with the code I have written would cause a null reference exception (other designs I have seen silently delete existing reviews if you don’t preload the collection).

In this case the C(r)UD only approach is better because it encapsulates the whole of the process so that calling methods don’t have to know anything about the process. Therefore, the C(r)UD only (and the C(r)UD & Business Logic) approach follows the Single Responsibility Principle, i.e. the AddReview is responsible for adding a new review to a Book, and entirely encapsulates the code to add that review.

However, the C(r)UD only approach has one big down side – it refers to EF Core. Why is this a problem? The entity classes, like Book and Review, are core parts of your domain design and Eric Evans says in chapter 6 of his book that repositories should be used to “decouple application and domain design from persistence technology”. There are many people who like having entity classes that contain no database access logic so that they the class is focused on domain (i.e. business) logic.

You have to remember that the repository pattern was the recommended approach when Eric Evan’s book was published in 2004. Personally I, and others, don’t think the repository pattern appropriate for EF Core (see my article “Is the repository pattern useful with Entity Framework Core?”).

In the next section we look more carefully at the business logic and how that is handled, but before I leave this comparison, I should talk about performance. In the two examples the POCO-only class approach is quicker, as it loads the reviews at the same time as the book, while the C(r)UD only approach needs two trips to the database. I used similar code two examples to make it easier for you to compare the two approaches, but in my actual code (shown below) I use a much more efficient approach if the _reviews collection is not loaded/initialised: It just creates a new review with the correct foreign key, which is much more efficient.

public void AddReview(int numStars, string comment, 
    string voterName, DbContext context) 
    if (_reviews != null)    
        //Create a new Book: the _reviews HashTable is set to empty
        _reviews.Add(new Review(numStars, comment, voterName));   
    else if (context.Entry(this).IsKeySet)  
        //Existing Book: create a new review directly
        context.Add(new Review(numStars, comment, voterName, BookId));
        throw new InvalidOperationException(
             "Could not add a new review.");  

3. Different ways of handling business logic

The first example was a Create, i.e. adding a new Review (known as an aggregate entity in DDD) to the Book entity (known as the root entity in DDD). These CRUD operations might include some form of validation, such as making sure the numStars value in the Review is between 1 and five. But when the rules get more complicated than validating properties then we tend to call the code business logic.

When someone wants an application written there are rules on how things work. For instance, placing an order might require certain checks on stock, payment etc. and kick off other tasks such as processing the order and delivery. The steps in the example order are called business rules, or in DDD-speak, domain problems.

In Eric Evans book he says “When the domain is complex, this is a difficult task, calling for the concentrated effort of talented and skilled people”. In fact, most of Eric Evan’s book is about domain problems: how to describe them (ubiquitous language), how to group domain problem (bounded context), etc. But in this section, I’m just going to look at the three DDD approaches and how they handle business logic.

For this I am going to use the example of processing a customer’s order for books. To make it easier to understand I am only showing 5 stages (e.g. I left out payment processing, shipment, etc.):

  1. Load the Books ordered using the primary keys provided by the higher layers
  2. Get the address that the order should be sent to
  3. Create the Order
  4. Save the Order to the database
  5. Send an email to the user saying the Order was successfully

I am also going to show the layering of the assemblies, i.e. in what order the assemblies reference each other. This matters because in .NET you can’t have circular references to assemblies (that causes problems at compile time), which constraints how you put the assemblies together. In most cases the entity classes are the lowest assembly, because everything else needs to access these key classes. (NOTE: I also add a red oval on the assembly that references EF Core).

2.a. POCO-only class approach

The POCO-only class approach uses a repository for database accesses so when the business logic needs data it relies on the repository. Because you can’t have circular references to assemblies this means part of the business logic moves into the repository. Typically, this results in repository taking on a coordination of the building of the order (Command Pattern). The figure to the right shows the assembly references.

And below is a diagram of one possible implementation of handling a customer order:

2.b The C(r)UD only approach

Over the years I have developed a series of rules to help me in implement business logic. And one of them is “Business logic should think it’s working on in-memory data”. Having decided that generic repository pattern doesn’t work well with EF Core (see my article “Is the repository pattern useful with Entity Framework Core?” on why that is) I had to come up with another way to do this. My solution is to have two new assemblies, which are:

  1. BizLayer: This contains classes that are pure business logic, i.e. they don’t access EF Core directly, but rely on their own mini-repository for all database actions
  2. BizDBAccess: This is handles all database accesses for a single business logic class, i.e. it’s a dedicated repository for the business logic.

It may seem odd to say I don’t like the repository pattern, and then build a dedicated repository for each business logic. But it’s because writing generic repository is actually very difficult – for instance EF Core is a generic repository/unit of work pattern and think how much effort it’s taken to write that! But writing a dedicated repository for a specific piece of business logic is easy. (I agree with Neil Ford’s comment from his book Building evolutionary architectures –The more reusable the code is, the less usable it is.”).

Now, I’m not going to show the code here, as I have a much longer article called “Architecture of Business Layer working with Entity Framework (Core and v6) – revisited” where I covers the same example of placing an Order (NOTE: this article doesn’t include a “SendMail” part, but I think you can see that goes in the business logic).

2.c. C(r)UD and business logic

The final version has methods/factories for ALL manipulation of the data in the entity class. This means any there is no repository and no business layer because it’s all handled by the entity class itself.

In this case the business logic code that was in the repository (see 2.a) is moved completely into the entity class. The technical issue with this is the SendMail method is in an assembly that is linked to the assembly containing the entity classes, which stops you referencing the SendMail directly. It’s pretty simple to fix this by defining an interface (e.g. ISendMail) in the entity class and using dependency injection to provide the SendMail instance at run time.

At this point every operation other that read (and maybe some deletes) will all be entity class, and those method will use EF Core to access the database directly from the entity itself.

Comparing approaches to business logic

In the end the differences are around the ‘scope’ of the code inside the entity class. The POCO-only has the simplest code, with no references to EF Core. The C(r)UD only approach uses EF Core to be a one-stop-shop for doing database changes but has the business logic in another assembly. While the C(r)UD + Biz Logic approach has everything that changes the database inside the entity class.

Here is a table where I try to list the advantages/disadvantages of each approach.

Approach Pros Cons
1. POCO-only Follows the original DDD description of handling entities. Hard to write generic repositories.

Repository pattern in data layer can cause performance problems.

The repository can be bypassed.

2. C(r)UD only Access methods follow the single responsibility principle.

Business logic is separated from database accesses.

More work writing the business logic and its dedicated repository.
3. C(r)UD + Biz Logic Everything in one place. Can suffer with the “God Object” anti-pattern.

Problems calling methods in “higher” assemblies.


While I think a DDD approach is useful, it’s a trade-off of features and limitations whichever way you go. In the end it depends on a) your application and b) what you feel comfortable with. For instance, if your application has some really complex business logic, with lots of calls to other parts of the system then the C(r)UD + Biz Logic approach might not work that well or you.


I do believe using a DDD approach with EF Core has many benefits – it can make the code clearer and more robust (e.g. no way to get it wrong). But coming up with a good DDD approach can take a while – I built my first DDD business logic approach in 2015, and it’s been through two iterations in 2016 and 2017 as I have improved it. This article is just another part of my continuing journey to learn and improve my skills around using DDD.

I am also want to be an efficient developer, so whatever approach must allow me to build quickly. But with a bit of help from some libraries, like the EfCore.GenericServices, I can build both robust, well-performing systems quickly. And when new features, like JSON Patch, become useful I try to adapt my approach to see if it can be added to my DDD style (see my article “Pragmatic Domain-Driven Design: supporting JSON Patch in Entity Framework Core” to see the outcome).

I hope this article helps you decide for yourself whether it’s worth using the DDD approach with EF Core and which approach suits your needs.

Happy coding!

Pragmatic Domain-Driven Design: supporting JSON Patch in Entity Framework Core

Last Updated: July 31, 2020 | Created: September 20, 2018

I’m a big fan of Domain-Driven Design (DDD) and build all my Entity Framework Core (EF Core) entity classes using a DDD approach, i.e. all properties have private setters and all changes are made via method in the entity class. This makes it very clear what, and how, properties and relationships can be updated. (See the article “Creating Domain-Driven Design entity classes with Entity Framework Core” for more details on implementing DDD in EF Core).

But there are times where a fully DDD-styled entity can create more work for yourself. I’m working with a client on a large application that will have hundreds of Web APIs feeding an Angular front-end. In this case using the JSON Patch feature will save the frontend developers a lot of time, because there are libraries that will process a form and create a JSON Patch to update the properties that have changes. (see the article “JSON Patch With ASP.NET Core“ for a fuller description of JSON Patch).

In this article I talk about how I merged the use of JSON Patch approach into my DDD entity class approach. I also describe how I added support for JSON Patch in my EfCore.GenericServices library, while keeping all the good attributes of a DDD-styled entity class.

Introducing the hybrid DDD entity class

JSON Patch, with its update via the property’s setters, seems at odds with the DDD approach to entity classes where properties have private setters. But you can’t ignore the fact that JSON Patch, and associated libraries, can significantly speed up the building of single-page applications, using Angular, React, Vue etc.

DDD is all about making the entity class business-focused, where it deals in terms of the business needs rather than simple properties. So, having methods in the entity class with names like MarkOrderAsDispatched is better than setting the OrderStatus to Dispatched. Also, DDD wants to provide the developer with a clear idea of if/how a property can be updated, especially around relationships.

When building my EfCore.GenericServices library I made it capable of updating “standard” entity classes, i.e. with public setters, and also work with DDD-styled entities, i.e. where updates are done via calling public methods. In all of this I did allow what I call a hybrid DDD entity class, where some properties were updated by method calls, and some were updated via their public setters.

The typical use case for a hybrid DDD entity class is updating an address, with its various street, city, state, zip/postcode properties, inside entity class. I wanted to keep DDD’s tight control over the relationships, but in many cases updating the address properties has no business effects so it simplified things to make the properties’ setters public (the library would notice that and call AutoMapper to update the properties from a DTO).

While some might say that a hybrid DDD entity class breaks the DDD approach, but I believe it follows the spirit of what Eric Evans was getting at in his book, Domain-Driven Design. The ‘intent’ of what can be done with property is clearly communicated.

  • If a property has a public setter then there are no business logic implications (other than validation).
  • If the property has a private setter then it either has business logic implications or you wanted to provide a business-named access method, like my earlier example of MarkOrderAsDispatched. Either way you must update it via a method call because the setter is private.

Altering a DDD-styled entity to allow JSON Patch to work

By default JSON Patch will fail on a property with non-public setter. That is actually useful, as we can differentiate between the properties that we allow JSON patch to update, and properties that should only be updated by ctors or methods. This is where the hybrid DDD entity class comes in.

Here is an example from my EfCore.GenericServices.AspNetCore library, which contains some simple entity classes and Web API to show how things work (you can clone and run this locally to try this out – it uses an in-memory which is seeded at startup to make it easier to play with).

I created a simple hybrid entity class called TodoItemHybrid, as shown below

public class TodoItemHybrid
    public int Id { get; private set; }

    [Required(AllowEmptyStrings = false)]
    public string Name { get; private set; }

    [Range(1, 5)]
    public int Difficulty { get; set; }

    //ctor and access methods
    public TodoItemHybrid(string name, int difficulty)
        Name = name;
        Difficulty = difficulty;

    public IStatusGeneric ChangeName(string name)
        var status = new StatusGenericHandler();
        if (name.EndsWith("!"))
            status.AddError("Business logic says the name cannot end with !", nameof(Name));

        Name = name;
        return status;

You can see that the Name property (line 6) has a private setter and can only be updated via the ChangeName method (lined 18 to 26). That method has some (fake) business logic connected to it (see lines 21 to 22), so that must be updated via the ChangeName method.

On the other hand, the Difficulty property (line 9) is a simple property with no business logic attached (apart from validation), so I have given it a public setter. This means I can update it via the Microsoft.AspNetCore.JsonPatch library.

The way that JSON Patch works is you can provide a list of changes. In this simple case there is only one property, Difficulty, but you can provide an array of updates. Here is simple example containing two updates to the same class:

	{ "op": "replace", "path": "/FirstName", "value": "Jon" }
	{ "op": "replace", "path": "/LastName", "value": "Smith" }

Using JSON Patch can cut down the amount of work required in the front-end code. A JavaScript library such as https://www.npmjs.com/package/rfc6902 can automate the creating of the JSON patches to send back to the server. This package has a createPatch feature that compares original data with new data (for instance, from a form) and generate a patch with all the changes to send to the server.

A look at the ASP.NET Core Web APIs

To apply such an update from a Web API controller, then you would need something like this

public IActionResult HandCodedUpdate(int id, 
     JsonPatchDocument<TodoItemHybrid> patch, 
     [FromServices]ExampleDbContext context)
    var entity = context.Find<TodoItemHybrid>(id);
    if (entity == null)
        return NoContent();
    return Ok();

The call sends the primary key of the entity you want to update (end of line 2), plus a JsonPatchDocument<T> patch (line 3) created by the caller. Then you have to load the entity you want to update (line 6) and if found you apply the patch and save the changes to the database (lines 11 to 12).

To make this more streamlined I have added a JSON Patch feature to my EfCore.GenericServices library. This wraps the ApplyTo with extra checks, optional validation and success/error messages. Here is the same example but using the GenericServices library:

public ActionResult<WebApiMessageOnly> Update(int id, 
    JsonPatchDocument<TodoItemHybrid> patch, 
    [FromServices]ICrudServices service)
    service.UpdateAndSave(patch, id);
    return service.Response();


DDD can provide many benefits, such as access methods which are named in a business-focused way, and these access methods are the definitive way to do something, as there is no other way in. I really like that level of isolation.

But most database tables have plenty of simple read/write columns that don’t need the level of access control. Writing access methods still works, but sometimes a simpler copy of data is sufficient. And when approaches like JSON Patch make the developers life simpler its worth adopting it in the right places.

In the end DDD is more a philosophy than a standard pattern – Eric Evans was saying “make the domain (business) problem the focus, not the tools and technology you use”. My hybrid DDD entity class might not be liked by some, but I think it still conveys the intent of the data, and it makes you a quicker developer.

Happy coding!

How to write good, testable ASP.NET Core Web API code quickly

Last Updated: July 31, 2020 | Created: August 7, 2018

This article is about ASP.NET Core 2.1 Web API code and ways to write code that is easy to understand, easy to test and quick to write. You might think that is quite a task, but I introduce a number of libraries that make that feat possible. Also there is an example application that you can clone from GihHub and run locally to see the libraries in action.

If you are new Web API I do start with a general introduction which covers the difference between Web APIs and using ASP.NET Core to directly deliver HTML pages. I then show you how to write simple Web API actions and set the correct HTTP response type back to the caller.

TL;DR; – summary

I describe a way to write Web API controllers that moves the database access code out to a library called GenericServices – this reduces the code needed in a Web API to a few lines. I also have another library called GenericBizRunner that does the same for business logic, which I cover in this article too. I describe a companion library called EfCore.GenericServices.AspNetCore that handles the creation of the correct response with a HTTP status code and json payload.

The overall effect of using the described pattern and libraries is that your Web API actions are small, easy to test and quick to develop.

NOTE: There is an example application on GitHub, which you can clone and run locally – it uses an in-memory database and includes Swagger, so its easy to run.

Introduction to ASP.NET Core Web API

Note: if you already know about ASP.NET Core Web API then you can skip this section.

There are now three main approaches to building a web application using ASP.NET Core version 2.1:

  1. NET Core MVC, which uses controllers, actions and views
  2. NET Core Razor Pages, which has pages with associated code
  3. NET Core Web API, which can be queried by a framework, like React.js, or an external system

The first two have the ASP.NET Core web app deliver HTML pages, with the help of the built-in Razor syntax. In these cases, the ASP.NET Core web app is in charge of what is shown on the screen – see diagram below:

But in the ASP.NET Core Web API case what is shown to the user is controlled by something outside ASP.NET Core. For the human-user example its either framework (like AngularJS or React.js) or mobile application makes requests to the ASP.NET Core application to either get data or execute some business logic – see the diagram below:

There are pros and cons to both approaches, but nowadays more applications are moving over some sort of front-end code running in the browser or mobile which uses Web API to access the backend system. Also, application-to-application traffic can also use the Web API approach. Therefore, being aware on how to write good, testable Web API code quickly is a useful skill.

Going beyond Microsoft’s Web API examples

Microsoft has some good examples of writing Web API applications, including one using EF Core for the database (VS Code version, VS2017/Win version and VS2017/Mac version). These are a great place to start as they clearly show what is going. Here is an example taken from Microsoft’s example (Note: I have left out the constructor of the Web API controller, where the EF Core DbContext for the database is injected and put in the private field called _context).

public IActionResult Update(long id, TodoItem item)
    var todo = _context.TodoItems.Find(id);
    if (todo == null)
        return NotFound();

    todo.IsComplete = item.IsComplete;
    todo.Name = item.Name;

    return NoContent();

The example is very clear about what is happening, as you can see each part of the process, which is why Microsoft wrote it that way. But when the databases accesses and business logic get more complex, then putting the code in the Web API action isn’t such a good idea, for a number of reasons:

  1. The code needed to achieve a particular action (in the example it’s a simple update) can become many lines of code, which means your controller is long and hard to understand.
  2. Your database or business logic is likely to have extra validation/checks which it needs to return as errors, which not covered in the Microsoft example.
  3. There is a danger of duplication of code. For instance, if another action needed to update the TodoItem you would you need to duplicate part of the code.
  4. It wouldn’t be hard to unit test the Web API action in Microsoft’s example, but in more complex applications with say extra data injected via tokens or other parts of the HttpContext then it can get more difficult.

Over the years I have learnt to keep my controller actions as small as possible. This means moving the code out of the action into a separate method. At the same time, I have noticed patterns in database accesses and business logic calls which have culminated in my building libraries to handle the common parts. Here is my version of the TodoItem update using my GenericServices library, which is designed to handle CRUD (Create, Read, Update and Delete) functions to a EF Core database.

public ActionResult<WebApiMessageOnly> Put(
    ChangeTodoDto dto,
    [FromServices]ICrudServices service)
    return service.Response();

The differences are:

  • Its short and to the point – my Web API controller will be small and easy to read
  • The library uses a general ‘update TodoItem’ method which is used throughout the application – no possibility of duplication of code.
  • My EfCore.GenericServices (injected via the [FromServices]ICrudServices service property on line 4) provides generic CRUD functions, including the mapping from the input class to the ToDoItem class.
  • The Response method on line 7 returns a response that is either OK (HTTP 200) with a success message, NoContent (HTTP 204) if the item wasn’t found, or if the update logic or database validation finds errors then it returns Invalid Input (HTTP 400) with the error messages (The format of these responses are shown later).

This approach also allows me to unit test the ‘update TodoItem’ on its own, which I prefer. As I said before testing Web API actions can work, but if you have complex HTTP things going on, like cookies or tokens providing data, then testing can get very hard. I find it much simpler and easier to test the service.UpdateAndSave(dto) code on its own in a unit test.

NOTE: If you are using EF Core then see my open-source library EfCore.TestSupport for which has lots of useful methods for testing code that uses EF Core. Also, consider using in-memory databases for unit testing as they are really quick – the article “Using in-memory databases for unit testing EF Core applications” to show you how.

So, by doing all this I have:

  • I wrote good code: is short, obvious and does not duplicate code.
  • I have written testable code: it’s easy to duplicate the same code in your unit tests.
  • I wrote the code quickly: The EfCore.GenericServices library handles the four CRUD patterns, including copying from a ViewModel/Dto class to the EF Core entity class.

It’s my EfCore.GenericServices open-source library that makes the code shorter and quicker to write. This library is a second-generation version of my original  GenericServices library built for EF6.x, and is significantly improvement over the original. My experience with the original GenericServices library was it has saved me months-per-year of development time. I expect my new library to give me a similar, if not better, productively as it now supports DDD-styled entity classes which centralises the create/update code.

I’m not going to describe the EfCore.GenericServices library in detail here, because I have already done that in the article “GenericServices: A library to provide CRUD front-end services from a EF Core database” (see also NuGet, GitHub, and documentation pages). But that article talks about MVC/Razor Pages usage.

In this article I am going to describe how to use my libraries with Web API, and to help I have updated the companion EfCore.GenericServices.AspNetCore library to convert the status output of my libraries library into the correctly formed HTTP responses.

What about business logic?

I have handled CRUD accesses to the database, but complex applications often have complex business logic. As I mentioned before I have another open-source library called EfCore.GenericBizRunner which runs business logic working with EF Core, which the companion EfCore.GenericServices.AspNetCore library also supports. You can find out more about the EfCore.GenericBizRunner library via the article called “A library to run your business logic when using Entity Framework Core”, and the GenericBizRunner’s documentation pages.

How I turn my libraries’ statuses into HTTP responses

My EfCore.GenericServices and EfCore.GenericBizRunner libraies are linked to EF Core, but could be used in any sort of application – web, desktop etc. But I mainly use them with ASP.NET Core, so I have a library called EfCore.GenericServices.AspNetCore which contains code to change the IGenericStatus that both these libraries use into either ASP.NET ModelState (for MVC and Razor Pages) into either

  • ActionResult<WebApiMessageOnly>, where it returns just a success message (or errors if incorrect).
  • ActionResult<WebApiMessageAndResult<T>>, if the Web API returns a success message and some results (or errors if incorrect).

In this article I want to look at the CreateResponse class that contain extension methods to turn my IGenericStatus into the two ActionResult forms.

NOTE: The EfCore.GenericServices.AspNetCore contains an ExampleWebApi project that contains an ASP.NET Core 2.1 Web API controller as an example of how the CreateResponse extension methods work. It uses an in-memory database and Swagger so that it’s easy to try out the Web API commands – just start the ExampleWebApi project and it should got directly to the swagger pages which lists each Web API command and allows you to test them.

Now I will describe the three main response types and what they look like. I start with the handling errors, as ASP.NET Core 2.1 has already defined that format.

InvalidInput (HTTP 400) – error return

If you create a new ASP.NET Core application the default template turns on 2.1 compatibility, which means that any Web API input is automatically validated, e.g. if a property had the attribute [Range(1,5)] then it would return an error if that property was outside that range. This is a really useful feature, as it saves you from having to add code to validate the input at the start of the Web API action.

The format that ASP.NET Core uses to return the errors is a dictionary of errors returned with Invalid Input (HTTP 400) status code. Here is an example of the format it uses:

    "": [
        "Global error message"

    "MyPropery": [
        "The property is required",
        "Another error on the same property"

Therefore, my Response extension method in the EfCore.GenericServices.AspNetCore library turns any errors found by the EfCore.GenericServices or EfCore.GenericBizRunner libray into the same format. That means there is one common error format the calling code needs to process.

The other responses we need are:

OK (HTTP 200) – successful

This contains a message string, which you can show the user or log. You can set the message yourself, or my libraries will return a message relevant to the. Also, if there is any data to return, then there is a results property. Here is an example with results, which uses the WebApiMessageAndResult<T> response class.

  "message": "Success",
  "results": {
    "id": 1,
    "name": "Create ASP.NET Core API project",
    "difficulty": 1

This type of response is has a result, as shown in this Get action method

[HttpGet("{id}", Name= "GetSingleTodo")]
public async Task<ActionResult<WebApiMessageAndResult<TodoItem>>>
    GetAsync(int id, [FromServices]ICrudServicesAsync service)
    return service.Response(
        await service.ReadSingleAsync<TodoItem>(id));

The other type of action that returns a result is a create. There are two ways to handle this.

1. Create, and return url

The standard Web API way of handling a create is to return a HTTP status of Created (201), with a url to get the newly created item. This library provides that feature, by linking into the ASP.NET Core’s CreatedAtRoute  method. Here is an example of this approach:

public ActionResult<CreateTodoDto> Post(CreateTodoDto item,
    [FromServices]IActionService<ICreateTodoBizLogic> service)
    var result = service.RunBizAction<TodoItem>(item);
    return service.Status.Response(this, "GetSingleTodo",
         new { id = result?.Id },  item);

NOTE: I have to allow for the result to be null, which can happens if there are errors. Therefore I use result?.Id  (see line 7) to stop me getting a null reference in the error state (thanks to Michael Wilson for spotting that).

On a successful creation this method returns a response header containing the URL you should use to obtain the newly created item. Here is an example of the header, with the url on the third line:

content-type: application/json; charset=utf-8
date: Thu, 30 Aug 2018 11:07:48 GMT
location: http://localhost:54074/api/ToDo/7
... other parts removed

NOTE: There is one “gotcha” that I need to warn you about. In ASP.NET Core you need to provide the Name of action method that provides the GET method to return the crated item. The Name isn’t the name of the method, but the name provided via the HttpGet attribute, for instance [HttpGet(“{id}”, Name= “GetSingleTodo”)]. If you don’t do that you get the dreaded “No route matches the supplied values” error.

2. Create, and return the created item

The other way to handle a Create is to return the created item. This has the advantage of removing the second HTTP to get the data. The library allows you to do this, and provide the 201 status code if required. Here is an example of a create a new TodoItem, where I return the created item, with its primary key:

public ActionResult<WebApiMessageAndResult<TodoItem>> Post(
     CreateTodoDto item,
     [FromServices]IActionService<ICreateTodoBizLogic> service)
    var result = service.RunBizAction<TodoItem>(item);
    return service.Status.ResponseWithValidCode(result, 201);

NOTE: You will see I use the [FromServices] attribute to allow the dependency injection (DI) provider to inject a service as a parameter in the action. I find this more efficient than the normal constructor injection, as it means each action creates just the service it needs.

NoContent (HTTP 204) – when the requested data lookup returned null

In Web APIs if something like a Get(id) is used, and no data is found (i.e. its null) then then it returns a status code of NoContent (HTTP 204). Therefore, if that happens then you get a NoContent status with a message, but no results. Here is an example of the json that is returned.

  "message": "The Todo Item was not found."

UPDATE: In version 2.0.0 of the EfCore.GenericServices.AspNetCore library I returned the NotFound status (HTTP 404) because Microsoft used NoFound status for nulls. But following feedback from others, and looking at other documentation, I released version 3.0.0 which returns a NoContent (HTTP 204) return on a null result. Version 3.0.0 also has a more fully defined return types, which helps Swagger to properly describe the API.

If you want to set your own Success and SuccessWithNullResult status you can the ResponseWithValidCode method, e.g. ResponseWithValidCode (result, 201, 404). The second parameter is the status code to return on a successful, non-null result, while the third parameter (which is optional – it defaults to 204) is the status code to return on a successful, but null result.

Telling GenericServices that null is not an error

To make the null handler to work you need to set a configuration parameter when you set up the GenericServices library. This is because in MVC or Razor Pages applications not finding the data asked for is an error, but in a Web API it isn’t. Here is an example of how you would configure GenericServices for WebAPI in the Configuration method in the Startup class – see line 4 for the important configuration option

    new GenericServicesConfig
        NoErrorOnReadSingleNull = true

UPDATE: Now with unit test support

I’m working on a client’s project which has lots of Web APIs, and we want to check them, because we found errors could creep up if the dtos weren’t properly setup.  I therefore added a new set of extension methods to the EfCore.GenericService.AspNetCore project (look for version 2.0.0 or higher on NuGet) to help test these.

The new extension methods decode the HTTP IActionResult and ActionResult<T> into a form that allows you to access all the parts of the information you sent from your GenericService/GenericBizRunner methods. This makes unit testing or integration testing much easier to do.

Here is an example integration test taken from the IntegrationTestToDoController, which you can find in the the EfCore.GenericService.AspNetCore GitHub project.

public void TestPutNameOk()
    var options = SqliteInMemory.CreateOptions<ExampleDbContext>();
    using (var context = new ExampleDbContext(options))

        var controller = new ToDoController();
        var utData = context.SetupSingleDtoAndEntities
        var service = new CrudServices(context, utData.ConfigAndMapper);

        var dto = new ChangeNameDto()
            Id = 2,
            Name = "Test",
        var response = controller.PutName(dto, service);

        var rStatus = response.CopyToStatus();
        rStatus.Message.ShouldEqual("Successfully updated the Todo Item");

Looking through the lines of code:

  • Lines 5 to 9: I create a in-memory Sqlite database a seed it with some known data for the test.
  • Line 11: I create an instance of the Web API controller I want to test. Because I inject the services via a parameter on the action method call, then there aren’t any parameters on the Web API controller.
  • Lines 12 to 14: Here I create the service that the Web API action method needs. The code you see is for GenericServices, but you can find an example of setting up GenericBizRunner here.
  • Lines 17 to 22: I call my Web API method, with the required dto class.
  • Line 25: The GetStatusCode extension method gets the HTTP status code, which I then check.
  • Line 26: The CopyToStatus method turns the Web API IActionResult/ActionResult<T> response back into a IStatusGeneric/IStatusGeneric<T> status (it started in from, so its easy to convert it back). This gives you access to any Errors, the Message, and the Result (if it has one).
  • Line 27 and 28 I check the parts of the IStatusGeneric to see what was send

NOTE: I could have added a test of the database to check that the name of the TodoItem with primary key 2 was changed too. It depends on whether you have unit tested the underlying GenericService UpdateAndSave of this entity elsewhere.

The extensions method can be found in UnitTesting.ResponseDecoders class.

NOTE: If you want to do a full integration test of your Web APIs then have a look at the articles “How to Test ASP.NET Core Web API” which uses the Microsoft.AspNetCore.TestHost package, or “Painless Integration Testing with ASP.NET Core Web API”  which uses Microsoft’s Microsoft.AspNetCore.Mvc.Testing package.


I have described the pattern and libraries that allow me to build good, testable Web API code quickly. Previous experience says that this approach makes me a much faster developer – I developed a MVC5 web application around the AdventureWorks 2012 Lite database in 10 days, which is quick (see http://complex.samplemvcwebapp.net/). Real applications that use Web APIs can have hundreds, if not thousands of Web API actions – saving even 25% development time on each Web API action could equate to a saving of many months of your time and the project’s time.

This article and the example Web API in the EfCore.GenericServices.AspNetCore GitHub repo give you a quick way to see the use of EfCore.GenericServices and I threw in an example of calling business logic via my EfCore.GenericBizRunner library (see the Post action which creates a new TodoItem) to build Web APIs.

The examples uses DDD-styled entity classes because I like the way it centralises the create/update code inside the entity class. Using DDD-styled entity classes is optional, as EfCore.GenericServices can work with normal EF Core entity classes as well, but I find it adds an extra level of the robustness of the final solution.

Do clone the the example code and look at the code and run the ExampleWebApi application – maybe using this approach will save you time too.

Thanks to the readers who left comments on the NotFound/NoContent argument! I have updated the EfCore.GenericServices.AspNetCore library to use NoContent, and better response definitions, which helps Swagger to define the API.

Happy coding!

Entity Framework Core – validating data and catching SQL errors

Last Updated: July 31, 2020 | Created: July 15, 2018

This article is about how to catch SQL errors, like unique index violations, and turning them into user-friendly error messages. This capability adds to the general area of validation of data being written to the database, especially around EF Core.

This article comes from needing this capability in my GenericServices and GenericBizRunner libraries while working on a project for a client. I also extended the design to handle concurrency issues in both libraries, as one of the users of my GenericServices library was asking for it.

TL;DR; – summary

When EF Core writes data out to the database it doesn’t validate that data (see Introduction to validation section for more on validation). However, a relational database will apply its own validation, such as checking that a unique index constraint hasn’t been violated, and will throw an exception if any constraint is breached.

The problem with database exception messages is that they aren’t user-friendly, and can reveal something about your database structure (which is a possible security breach), so you can’t show them directly to the user. This article describes how to capture these database exceptions in EF Core and turn them into user-friendly error messages.

Introduction to validation in EF

Note: if you know about data validation and how EF Core does (or doesn’t) validate data on save then you can skip this section.

Data validation in .NET is about checking that the data in a class fits within certain rules. Typical validation rules are things like the attribute [MaxLength(100)] on a string. Other more complex validation rules can be applied via the IValidatableObject Interface. Some of these attributes, like [MaxLength(…)], [Required], etc. are also used by EF to set the size, nullability etc. of columns in the database.

Any validation rules are typically checked at the front end, say in ASP.NET Core, but can also be checked when that data is saved to the database. The default state for EF6.x is that the data written to the database is validated, but in EF Core the data isn’t validated – the reasoning is that the data  its most likely been validated earlier, so leaving out validation makes the save quicker.

Personally, when I create data in a business logic I always validate the data, as no front-end checks have been applied and the business logic might get things wrong. This is why my GenericBizRunner library defaults to validating the data on save, while my GenericServices library, which works with the front-end CRUD (Create/Read/Update/Delete) accesses, defaults to not validating the data.

But as I said at the beginning, whether you validate the data or not the relational database will have some constraints that it will always apply – you can’t bypass them for the good reason that the database wants to keep its state valid at all times. If you breach a database constraint in EF6.x or EF Core you will get an exception.

How to capture a database exception in EF Core

When you call SaveChanges/SaveChangesAsync then a range of exceptions can occur, from EF Core caught issues like concurrency issues (DbUpdateConcurrencyException, to database issues like the example of the unique index violation (DbUpdateException). Here is an example of catching a DbUpdateException taken from my book “Entity Framework Core in Action” (section 10.7.3).

catch (DbUpdateException e)
   //This either returns a error string, or null if it can’t handle that error
   var error = CheckHandleError(e);
   if (error != null)
      return error; //return the error string
   throw; //couldn’t handle that error, so rethrow

In the case of the DbUpdateException has an inner exception, which is the actual database exception containing the information about the constraint violation. The type and content of this inner exception differs for each database type.

The other thing to note is that the calling method has to return either success, or one or more error messages.

My generalized SaveChangesExceptionHandler

For my libraries I wanted to generalise the approach to cover both a) any type of exception, b) any type of databases. The signature for the SaveChangesExceptionHandler is

Func<Exception, DbContext, IStatusGeneric>

The method takes in the exception to decode and returns either a status, which can have errors or a success message, or null if it doesn’t handle the error type it was given. I also supply the current DbContext, which you would need if you were trying to handle a DbUpdateConcurrencyException.

Example exception handler targeting SQL Server

For my example I am going to show you a exception hander that works for SQL Server, but the approach works for any database – you just have to decode the database errors from the inner exception specific to your database.

The code below is written to capture the inner SqlException and only handles SQL unique index violations. It starts with the method that detects each type of error you want to handle, and then calling the appropriate method. It’s basically a sophisticated switch pattern, and you can add more error handling by adding extra tests.

public const int SqlServerViolationOfUniqueIndex = 2601;
public const int SqlServerViolationOfUniqueConstraint = 2627;

public static IStatusGeneric SaveChangesExceptionHandler
    (Exception e, DbContext context)
    var dbUpdateEx = e as DbUpdateException;
    var sqlEx = dbUpdateEx?.InnerException as SqlException;
    if (sqlEx != null)
        //This is a DbUpdateException on a SQL database

       if (sqlEx.Number == SqlServerViolationOfUniqueIndex ||
           sqlEx.Number == SqlServerViolationOfUniqueConstraint)
          //We have an error we can process
          var valError = UniqueErrorFormatter(sqlEx, dbUpdateEx.Entries);
          if (valError != null)
              var status = new StatusGenericHandler();
              return status;
          //else check for other SQL errors

    //add code to check for other types of exception you can handle

    //otherwise exception wasn't handled, so return null
    return null;

Having found the unique index errors I call a method to specifically handle that sort of error. I should say that I add a constraint/key name following the pattern ‘UniqueError_<EntityName>_<PropertyName> to any unique indexes or primary keys that I want to handle. This both gives me more info to show the user, and stops me trying to handle errors on properties I didn’t expect to have an error. Here is my UniqueErrorFormatter method.

private static readonly Regex UniqueConstraintRegex =
    new Regex("'UniqueError_([a-zA-Z0-9]*)_([a-zA-Z0-9]*)'", RegexOptions.Compiled);

public static ValidationResult UniqueErrorFormatter(SqlException ex, IReadOnlyList<EntityEntry> entitiesNotSaved)
    var message = ex.Errors[0].Message;
    var matches = UniqueConstraintRegex.Matches(message);

    if (matches.Count == 0)
        return null;

    //currently the entitiesNotSaved is empty for unique constraints - see https://github.com/aspnet/EntityFrameworkCore/issues/7829
    var entityDisplayName = entitiesNotSaved.Count == 1
        ? entitiesNotSaved.Single().Entity.GetType().GetNameForClass()
        : matches[0].Groups[1].Value;

    var returnError = "Cannot have a duplicate " +
                      matches[0].Groups[2].Value + " in " +
                      entityDisplayName + ".";

    var openingBadValue = message.IndexOf("(");
    if (openingBadValue > 0)
        var dupPart = message.Substring(openingBadValue + 1,
            message.Length - openingBadValue - 3);
        returnError += $" Duplicate value was '{dupPart}'.";

    return new ValidationResult(returnError, new[] {matches[0].Groups[2].Value});

There is some weird regex and decoding of the SQL error message, and that is because the error string isn’t that simple to decode. Here is an example error message:

Cannot insert duplicate key row in object ‘dbo.Books’ with unique index ‘UniqueError_Book_ISBN’. The duplicate key value is (9781617294563).

The UniqueErrorFormatter method reformats this into a message

Cannot have a duplicate ISBN in Book. Duplicate value was ‘ 9781617294563’.

Notice on lines 13 to 15 I try to find the entity class so that I can return a user friendly and localized name, but the name in the constraint/key name is pretty good (unless you have a TPH class).

The SaveChangesWithValidation code

The code above is called within my SaveChangedWithValidation method, which I show below. The method returns the IStatusGeneric interface I use in my libiraries. This contains either a success message if there were no errors, or a list of errors if problems were found.

public static IStatusGeneric SaveChangesWithValidation(this DbContext context, IGenericServicesConfig config)
    var status = context.ExecuteValidation();
    if (!status.IsValid) return status;

    context.ChangeTracker.AutoDetectChangesEnabled = false;
    catch (Exception e)
        var exStatus = config?.SaveChangesExceptionHandler(e, context);
        if (exStatus == null) throw;       //error wasn't handled, so rethrow
        context.ChangeTracker.AutoDetectChangesEnabled = true;

    return status;

The ExecuteValidation code looks like this

private static IStatusGeneric ExecuteValidation(this DbContext context)
    var status = new StatusGenericHandler();
    foreach (var entry in
            .Where(e =>
                (e.State == EntityState.Added) ||
                (e.State == EntityState.Modified)))
        var entity = entry.Entity;
        var valProvider = new ValidationDbContextServiceProvider(context);
        var valContext = new ValidationContext(entity, valProvider, null);
        var entityErrors = new List<ValidationResult>();
        if (!Validator.TryValidateObject(
            entity, valContext, entityErrors, true))

    return status;

NOTE: You can find all of this code in the class SaveChangesExtensions in the GenericServices library.

Enabling this feature in my GenericServices and GenericBizRunner

You need to configure the libraries to take your error handler. GenericBizRunner is the simplest as it already defaults to validation on writes to the database. Here is the configuration code.

   new GenericBizRunnerConfig
    SaveChangesExceptionHandler = 

The GenericServices is a bit more complex, as by default it doesn’t validate on saving to the database, so your error handler won’t be called. You can turn on validation on a case-by-case via the PerDtoConfig<TDto, TEntity> class, or turn on validation for all writes to the database via the global configuration, as shown below.

new GenericServicesConfig
        DtoAccessValidateOnSave = true,     //we use  Dto access for Create/Update
        DirectAccessValidateOnSave = true,  //And direct access for Delete
        SaveChangesExceptionHandler = GenericServiceErrorHandler.SaveChangesExceptionHandler
}, Assembly.GetAssembly(typeof(MyDto)));

Other things you could do with this

As I said earlier one of my users of the GenericServices library wanted to know if GenericServices could handle concurrency issues, and I had to say no. But, now with my generalised SaveChangesExceptionHandler both libraries can handle concurrency issues. I haven’t done this yet, but the steps would be:

  • In the ‘switch’ method you would detect that the exception was a DbUpdateConcurrencyException and the type of the entity it happened on. Then you would call a method designed to handle concurrency issues on that entity type.
  • In that method you would (try) to fix the issue. If you have then you need to call SaveChanges (within another try/catch in case the error happened again) to save the corrected update.
  • If successful you return a status of success, which the system would return to the user. But if there were errors it couldn’t handle it can report to the user that the process failed.

NOTE: that handling concurrency issues is quite complex and hard to test, so I’m not saying this is simple. But you can now do this in in my GenericServices and GenericBizRunner libraries.

NOTE: I cover this concurrency issues in detail in section 8.7 in my book – it takes 12 pages!


Catching SQL errors and turning them into user-friendly error messages is hard work, but in real applications its often the only way to ensure the system works. Otherwise the user will be presented with the rather unhelpful “There was an error” page. Certainly not acceptable in real applications.

I spent bit of my own time to generalised the error handler in my GenericServices and GenericBizRunner libraries to cover more possible exceptions and work with any database. Now you have access to this feature in both of my libraries, or you can copy my approach in your own applications.

I hope this helps you.

Happy coding!