Create Dynamic <T> in C#

Question

I need to create Dynamic T using interface. But I am getting "Type Casting" error. Here is my code :

interface IEditor { }

class Editor : IEditor { }

class Test<T> { }

Now the will be dynamic so I am using this code below :

Test<IEditor> lstTest = (Test<IEditor>)Activator.CreateInstance(typeof(Test<>).MakeGenericType(typeof(Editor)));

I am getting following error

Unable to cast object of type 'CSharp_T.Test`1[CSharp_T.Editor]' to type 'CSharp_T.Test`1[CSharp_T.IEditor]'.

This error is not compilation error but I am getting run time error.

Answer 1

Generic classes do not support covariance, but interfaces do. If you define an interface ITest<> and mark T as an out parameter, like this,

interface IEditor { }

class Editor : IEditor { }

interface ITest<out T> { }

class Test<T> : ITest<T> { }

you will be able to do this:

ITest<IEditor> lstTest = (ITest<IEditor>)Activator
    .CreateInstance(typeof(Test<>)
    .MakeGenericType(typeof(Editor)));

However, this would limit the ways in which the T parameter could be used inside ITest<> and its implementations.

Demo on ideone.

Answer 2

Test is not covariant (it is invariant in it's generic argument). Because of this a Test<IEditor> is not a subtype of a Test<IEditor>. There is no relationship between those two types.

You can create an object of type Test<IEditor> to begin with, instead of a Test<IEditor>, and then the cast can succeed.