Cannot inherit from generic base class and specific interface using same type with generic constraint

Question

Sorry about the strange title. I really have no idea how to express it any better...

I get an error on the following snippet. I use the class Dummy everywhere. Doesn't the compiler understand the constraint I've added on DummyImplBase? Is this a compiler bug as it works if I use Dummy directly instead of setting it as a constraint?

Error 1 'ConsoleApplication53.DummyImplBase' does not implement interface member 'ConsoleApplication53.IRequired.RequiredMethod()'. 'ConsoleApplication53.RequiredBase.RequiredMethod()' cannot implement 'ConsoleApplication53.IRequired.RequiredMethod()' because it does not have the matching return type of 'ConsoleApplication53.Dummy'. C:\Documents and Settings\simen\My Documents\Visual Studio 2008\Projects\ConsoleApplication53\ConsoleApplication53\Program.cs 37 27 ConsoleApplication53

public class Dummy
{
}

public interface IRequired<T>
{
    T RequiredMethod();
}

public interface IDummyRequired : IRequired<Dummy>
{
    void OtherMethod();
}

public class RequiredBase<T> : IRequired<T>
{
    public T RequiredMethod()
    {
        return default(T);
    }
}

public abstract class DummyImplBase<T> : RequiredBase<T>, IDummyRequired
    where T: Dummy
{
    public void OtherMethod()
    {
    }
}

Answer 1

I you change the DummyImplBase to:

public abstract class DummyImplBase<T> : RequiredBase<Dummy>, IDummyRequired
        where T : Dummy
{
    public void OtherMethod()
    {
    }
}

Answer 2

You could add this to DummyImplBase:

   public Dummy RequiredMethod() { return base.RequiredMethod(); }

EDIT: Or, if you're using C# 4.0, you could change definition of IRequired like so:

public interface IRequired<out T>

then remove IDummyRequired and you would still be able to assign derived classes to IRequired<Dummy> (but not to IDummyRequired).

2nd EDIT: Your original code did not compile, because T could be Dummy or a class derived from Dummy. And the implemented Method

public DummyDerived RequiredMethod() 

has not the same as signature as

public Dummy RequiredMethod()

which was declared in IDummyDerived.

Answer 3

If I make IDummyRequired generic too it works, so I guess I'll have to change some of my design to make this possible. I'm still hoping to find out why my solution doesn't work though.

public class Dummy
{
}

public interface IRequired<T>
{
    T RequiredMethod();
}

public interface IDummyRequired<T> : IRequired<T>
    where T : Dummy
{
    void OtherMethod();
}

public class RequiredBase<T> : IRequired<T>
{
    public T RequiredMethod()
    {
        return default(T);
    }
}

public abstract class DummyImplBase<T> : RequiredBase<T>, IDummyRequired<T>
    where T: Dummy
{
    public void OtherMethod()
    {
    }
}