Question

I have a little problem for you guys.

I would like to do this:

Type[] classes = new Type[]{ Class1, Class2 };

foreach(Type t in classes){   
  List<t> list = new List<t>(); 
}

Is there some way to do this?

Answer 1

You cannot cast to a generic type at runtime, because the type in the generic needs to be resolved at compile time.

You can create a generic type in a dynamic manner using reflection, but unless you hard-code the cast, all you get in return is an object.

I cannot really tell what it is you want, I have to agree with a comment this is an XY problem. I would be inclined to make the presumptuous statement that there is a design issue somewhere that this is trying to solve, instead of addressing the design issue directly, or asking the question of what you are trying to achieve directly.

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You can use the following code to create the type, then the dynamic type can be used to duck type the various members of List<T> without knowing/caring that it is a list or what T is:

using System;
using System.Collections.Generic;

namespace ConsoleApplication61
{
    class Program
    {
        static void Main(string[] args)
        {
            dynamic o = CreateGeneric(typeof(List<>), typeof(int));
            o.Add(1);
            Console.WriteLine(o[0]);
            Console.Read();
        }

        public static object CreateGeneric(Type generic, Type innerType, params object[] args)
        {
            System.Type specificType = generic.MakeGenericType(new System.Type[] { innerType });
            return Activator.CreateInstance(specificType, args);
        }
    }
}

The above sample duck types the Add method and the Indexer. The DLR does the type handling and the duck typing at runtime – knowing that 1 is an int, for example.

Just to clarify, I likely wouldn’t use such code in production (unless you requirements are very specific to need this) and any issues with type-mismatching will occur at run time; so you either need to type very accurately (limited IntelliSense) or have good error handling.

Thanks to this blog post for the CreateGeneric method.

This assumes .NET 4 with the new CLR. As @MartinLiversage has also pointed out, this particular sample assumes that you are utilising the list in a sort-of-strongly-typed manner. In my example I am passing an int to a List<int> hidden in a dynamic.

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We have been on .NET 4 almost since it was released. We have a large application with an even larger code base. dynamic isn’t used once in the application, and only a few times in the test code base. That isn’t to say “don’t use it”, it’s to say “most of the time, you don’t need it”.