.net 4 generics question

Question

Hi, I have the following class structure:

public class A : AInterface { }
public interface AInterface { }

public class B<T> : BInterface<T> where T : AInterface 
{
    public T Element { get; set; }
}
public interface BInterface<T> where T : AInterface 
{
    T Element { get; set; }
}

public class Y : B<A> { }

public class Z<T> where T : BInterface<AInterface> {}

public class Test
{
    public Test()
    {
        Z<Y> z = new Z<Y>();
    }
}

This gives me the following compile erorr in C# 4.0. The type 'Test.Y' cannot be used as type parameter 'T' in the generic type or method 'Test.Z'. There is no implicit reference conversion from 'Test.Y' to 'Test.BInterface'.

I though the covariance in generics should make this work? Any help will be appreciated.

Answer 1

I think you are missing the out keyword. Try adding it to the following lines:

public interface BInterface<out T> where T : AInterface { }
public class Z<out T> where T : BInterface<AInterface> {}

I'm not sure if it's needed in both places, though.

Answer 2

Generic parameters in interfaces are invariant by default, you need to explicitly specify whether you want a particular generic parameter to be covariant or contravariant. Basically, in your example you need to add "out" keyword to the interface declaration:

public interface BInterface<out T> where T : AInterface { } 

You can find more info about creating variant interfaces on MSDN: Creating Variant Generic Interfaces (C# and Visual Basic).