Using .NET Generics with a type derived at runtime

Last Updated: July 31, 2020 | Created: February 7, 2015

I have used .NET Generic types a lot and found them really useful. They form a core part of the solution in my open-source GenericServices library and my private GenericActions library. This article explores why and how I use Generics and some of the techniques I use to hide the more complex type definitions.

Even if you are not interested in my particular use of Generics this article has some useful information on:

  1. How to create an instance of a generic type where the type is defined at runtime.
  2. The two approaches to calling methods or read/write properties of the created generic instance.
  3. The performance issues around the two approaches to calling methods or read/write properties.

Only interested in how to use Generics with a runtime-derived type? Just skip to here in this article.

What are Generic Types

Generics are everywhere – if you use List<T> etc. you are using Generics. If you are not familiar with Generics then look at this introduction. The primary thing about Generics is it allows you to write one code implementation that can then work on a range of types. In particular they produce efficient code because they are ‘type safe’, i.e. the type is known, so the code can access what it needs.

Without Generics we would either:

  • Have to write the same code for each type, which is bad practice.
  • Write one method but use Reflection to access the items we want, which is not type-safe.

The down side of Generics is you (normally) have to know the type at compile time. However I will show you later how to overcome this.

Where I have found Generic types really useful

I developed a web application called Spatial Modeller that does mathematical predictive modelling for healthcare. The modelling is complex and I used a  Domain-Driven Design (DDD) approach, which said I should keep the database/business classes focused just on the modelling problem. That worked really well, but it meant that the database/business classes weren’t right for display to the user.

I therefore had a Service Layer which had classes more attuned to the display and visualisation of the data. These are typically called Data Transfer Objects (DTOs), or in ASP.NET MVC they are also called ViewModels. The Service Layer code transformed the data passing between the Data/Business Layers and the Presentation/WebApi Layer.

The problem was I ended up with writing very similar code, but with different data types, for the transformation of the database/business classes to DTOs. That is both bad software practice, time consuming and boring!

After that project I was determined to create a solution for that. It took a bit of experimenting, but Generics was the answer. However the final solution wasn’t quite a straightforward as you might think.

The problem of a complex Generic type definitions

Let me give you a real example of where my presentation layer wants to run a piece of business logic. In Spatial Modeller to model a scenario the following happens:

  1. The user picks the potential hospital locations for dropdown lists and fills in various selections.
  2. These are handed to the business layer modeller as primary keys for the hospital locations and various enums/parameters.
  3. The modeller does its work, writing some data to the database.
  4. The modeller returns the results as primary keys.
  5. These need to be looked up show the user.

So, in that process we have five classes:

  1. The presentation input DTO, called DtoIn.
  2. The business modeller data input, called BizIn.
  3. The class/method to call in Business Layer, called IBizAction.
  4. The output of the business modeller, called BizOut
  5. The presentation output DTO, called DtoOut.

So, if we define a Generic class to handle this its class definition would look like this:

var service = new ActionService<DtoIn, BizIn, IBizAction, BizOut, DtoOut>
     (db, new BizAction(db));
var dataIn = new DtoIn(some data here );
var result = service.RunAction(dataIn);

That is a) quite hard to write and b) not easy to use with Dependency Injection and c) downright ugly! In fact in both of my libraries its even more complex than that, with different types of input. Even worse the code will be the same, but have different transform parts. In GenericActions I calculated there were sixteen versions, all of which would need separate implementations.

GenericServices was much easier, but still produced ‘ugly’ generic type definitions, which needed special handling at dependency injection time.

Getting rid of the ‘Ugly’ type definitions

OK, so Generics got me so far but I needed to do something else to combine and simplify the type definition of the Generics. I did two things:

  1. For both libraries I hide the complex type definition via a outer, non-generic helper class where the called method does a decode of the types and creates the correct Generic class instance with the right types.
  2. In the case of GenericActions, which has so many versions, I did a further decode of the in/out types inside the Service Layer code, which reduced the sixteen possible versions down to six. I could have gone further, but this was the right level to keep performance up.

Before I describe the solution I will show you the improved code for the example in the last section (see below). I think you will agree that is much nicer, and the creation of the service is now very Dependency Injection friendly.

var service = new ActionService<IBizAction>(db, new BizAction(db));
var dataIn = new DtoIn(some data here);
var result = service.RunAction<DtoOut>(dataIn);

So, let us go through the steps to achieve this.

Part 1: Decode the underlying types

I’m going to use an example from GenericServices, as it is open-source and I can therefore link to the actual code. So the example for GenericServices is:

var service = new UpdateService(db);
var dataIn = new DetailPostDto(some data here);
var response = service.Update(dto);

The steps are:

  1. service.Update<T>(dto) is a top-level method (see first class) that can take either a database class, like Post, or a dto.
  2. If it is a dto then it is derived from abstract EfGenericDto<TEntity, TDto>. This forces the dto definition to include the database class. See the DetailPostDto.cs as an example.
  3. The Update method calls DecodeToService<UpdateService>.CreateCorrectService<T> (see this class, first method). It gets a bit complicated in this class because of sync/async versions of DTO, but the bottom line is it:
    1. Finds if it is a EfGenericDto or not. If not then it assumes its a direct access.
    2. If it inherits from EfGenericDto it finds the database class and the Dto class.


Part 2: Create the Generic Class from the runtime types

Ok, the last process found whether it was a database class, or a dto class. There are different classes to handle these two cases. The direct update has one Generic Type parameter, being the database class. The dto version has two Generic Type parameters: the database class and the dto class. However in the example below I look at a very simple case to make it easier to understand. GenericServices is a bit more complicated, but follows the same approach.

The code to create the instance of the Generic class is petty straightforward. For example if I wanted to create a List<string> at runtime I would

  1. Produce an array of the type(s) need to form the generic type, in this example ‘string’
  2. Get the generic type, in this example ‘List’
  3. Combine them using the ‘.MakeGenericType’ method
  4. Create an instance of that type using ‘Activator.CreateInstance’

The code below shows an example of creating ‘List<string>’ at runtime.

var dataType = new Type [] { typeof(string)};
var genericBase = typeof(List<>);
var combinedType = genericBase.MakeGenericType(dataType);
var listStringInstance = Activator.CreateInstance(combinedType);

Part 3: Calling methods from the created instance

You should be aware that ‘Activator.CreateInstance’ method returns an object, so you can’t just ‘listStringInstance.Add(“hello world”)’ as the compiler will something like “‘object’ does not contain a definition for ‘Add’“. You have two choices:

1. Use Dynamic Type

You can place the output of the ‘Activator.CreateInstance’ into a dynamic type (see start of line 4). This turns off compile time type checking which allows you to call any method, or access any property, with the type checking done at runtime. So in our List<string> case it would look like this:

var dataType = new Type [] { typeof(string)};
var genericBase = typeof(List<>);
var combinedType = genericBase.MakeGenericType(dataType);
dynamic listStringInstance = Activator.CreateInstance(combinedType);
listStringInstance.Add("Hello World");

Dynamic is easy to use, and allows much better freedom. However the dynamic runtime library takes a lot of time on the first call of the method. See Performance section for more detailed analysis.

2. Use Reflection

Reflection allows you to find methods, properties by name. You can then call the method or access the property via different Reflection methods. In our example we are calling a simple method ‘Add’ in the instance type (would be null if no method of that name existed) and then ‘Invoke’ that method, i.e.

var dataType = new Type [] { typeof(string)};
var genericBase = typeof(List<>);
var combinedType = genericBase.MakeGenericType(dataType);
var listStringInstance = Activator.CreateInstance(combinedType);
var addMethod = listStringInstance.GetType().GetMethod("Add");
addMethod.Invoke(genericInstance, new object[]{"Hello World"});

The reflection approach does have some complications, such as method or property accesses returns an object, which can be a problem if you need to type it. Also if the method had an output you need to build a generic Method using ‘MakeGenericMethod’. All in all reflection is harder to handle that dynamic.

However the reflection approach has a tiny first-use cost compared to the dynamic approach. However for lots of subsequent calls to the method then dynamic is quicker. See Performance section for more detailed analysis. (below)

Performance issues – simple types

The way you access the method or property does have a big effect on the performance of the command, as we are doing more. I care about performance so I have studied this in detail. There are two costs, one-off  first-use compute time and per instance compute time.

Let me start with a simple table giving the times for my simple List<string> example. What I did was run the same test on three types:

  1. Normal code, i.e var list = new List<string>(); list.Add(“Hello World”);
  2. Using reflection, i.e. listStringInstance.GetType().GetMethod(“Add”);
  3. Using dynamic, i.e. listStringDynamicInstance.Add(“Hello World”);

To take out any first-use costs I ran it on List<string> twice, followed by List<DateTime> three times, as it did seem to change. The List<DateTime> is there to check if building a different type has the same first-use cost. Here are the results, which were measured using the ANTS Profiler.

BE WARNED: This table is a bit misleading by implying refection is always faster than dynamic – it isn’t. See the ‘Performance issues – GenericAction library’ section where a real-life example shows that dynamic wins in the end.

Type Order Compiled Reflection dynamic
List<string> 1. First 0.8 ms 37.08 ms 5.0 ms
(was 600ms)
List<string> 2. Second < 0.001 ms 17.05 ms 1.0 ms
List<string> 2. Third < 0.001 ms 0.01 ms 0.6 ms
List<DateTime> 3. First (after List<string>) < 0.001 ms 0.03 ms 2.7 mS
List<DateTime> 4. Subsequent < 0.001 ms 0.03 ms 0.5 ms


When I first started testing I was getting 600ms for the first-use cost on the first dynamic method call, of which about 400 ms comes from the Dynamic Language Runtime. However once I installed .NET 4.5.2 on my windows system this dropped to 5ms. I cannot confirm that as the release notes do not say there is a change there. Maybe it was just reinstalling the .NET library. Anyway, be warned that things might have changed.

The way that .NET handled dynamic types is complex and I don’t claim to understand it properly. I would refer you to Simon Cooper’s series on ‘Inside DLR’ (Dynamic Language Runtime) starting with ‘Inside the DLR – Callsites‘ and then look at ‘Inside DLR – Invoking methods’ which describes the caching of methods.

Performance issues – GenericAction library

In GenericServices and GenericActions I have used the dynamic approach. Seeing the figures above made me wonder if that was the best way. I therefore forked a version of GenericActions and changed it to use a Refections approach with no use of dynamic anywhere. Here are my findings:

  1. When comparing the dynamic-based and the Reflection-based versions of GenericActions the first-use costs of dynamic are much less than the table above suggests. The figures are:
    1. Refection-based: 300 ms first-use cost due to AutoMapper first-use costs.
    2. dynamic-based: 700 ms first-use cost, which, if you exclude the AutoMapper part, means that the dynamic part is only 400 ms. I haven’t measured after installing .NET 4.5.2 but it now seems quicker.
  2. If a created instance is used many times then dynamic wins in the end. As an example my test of system on a 1000 accesses then dynamic was between 120% and 200% faster than reflection (on my tests anyway).
  3. Dynamic is much easier to use. I found it really hard to do everything with reflection, mainly because you cannot cast something. My altered GenericActions version worked, but some of the error checking on incorrect types did not work any more. I might have fixed them, but it wasn’t worth it.

So, lets look at the different parts of the performance problem.

1. One-off first-use compute time costs

With dynamic there is a cost on first decode and call of the method in my libraries which use dynamic – about 0.2 seconds or more on my system. As I explained earlier this because the first time you call a method in a dynamic type its needs to be created, which takes some time. However the method is then cached so later calls are very fast.

Any first-use performance cost is a pity, but I have other first-use time costs, like MVC, Entity Framework, AutoMapper etc., so I accepted , mainly because I can mitigate it (see next section) and overall the dynamic approach is faster.

Mitigating first-use costs

One thing that really helps is keeping the application alive so that the first-use cost only happens when you start/restart your application. I mainly develop web applications and one great feature introduced in .NET 4.5.1 was ‘Suspend‘ on an ASP.NET web site.

On shared hosting (maybe other hosting as well, not sure) when a web site has no traffic, i.e. all user sessions have timed out, then it stops. This means for low-traffic sites the web application would stop and the next user coming to it would have to wait for it to start. The ‘Suspend’ feature keeps the application alive so that this does not happen.

Suspend support does depend on your hosting provider setting up the feature in IIS so please read this post on it. My non-Azure shared hosting provider WebWiz supports this. Azure web sites doesn’t support ‘Suspend’ but fixes this with an ‘Always On’ feature which polls the site regularly enough that it stays up.

2. Per instance compute time

The whole decode and create takes about 0.0024 ms on my system, excluding first-use compute time. The actual creation of an instance of the generic type isn’t that costly (I haven’t measured it separately), but it is the decoding of the types etc. that take the time.

In the case of GenericServices the database call is so large, > 1 millsecond in all measured cases, that its not worth trying to improve the decode/create time.

However in GenericActions, which may call a very simple business method which could return very quickly, it is worth looking at. I therefore implemented a system caching system using a ConcurrentDictionary in my GenericActions library. It took quite a bit of tuning but the effect was worthwhile, as it brought the worse case, which has multiple decode calls, down from 0.037 ms to 0.018 ms. These are small figures, but I think worth the effort to make calls to the business logic have a low performance cost.


I have described not only a way of creating a Generic Type from a runtime type, but a whole system for using complex Generic Types, yet still having type definitions that are simple to use and Dependency Injection friendly. I have also looked how you call the methods or access the properties in the runtime created type, focusing on ease of use and detailed performance figures. I hope that is useful to people.

Happy coding!

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