Question

I have a set of classes and interfaces which have a relatively simple hierarchy, however having things inherit and then having a variable which accepts the base class is not working well. I think the problem is that my dynamic types MUST be where : class and not where : IEntity because these types are used in functions which I have no ability to change.

public interface IB<X, Y> 
    where X : class, IEntity 
    where Y : class, IEntity
{
    ...
}

public abstract class A
{
    ...
}

public abstract class B<X, Y> : A, IB<X, Y> 
    where X : class, IEntity 
    where Y : class, IEntity
{
    ...
}

public interface IEntity
{
    ...
}

public class Entity1 : IEntity
{
    ...
}

public class Entity2 : IEntity
{
    ...
}

public class C : B<Entity1, Entity2>, IB<Entity1, Entity2>
{
   ...
}

And then, attempting to use all this in a function…

C c = new C();
IB<IEntity, IEntity> ib = c;

Cannot implicityly or explicity cast this.

How can I get this to work?

Answer 1

Your interface is not covariantly valid. That is to say,

IB<IEntity, IEntity> and IB<Entity1, Entity2> are not the same thing. To fix this, you can use the out keyword on the type parameters in the interface definition

public interface IB<out X, out Y> where X : class where Y : class

As a secondary point, you need to cascade your constraints to class B

public abstract class B<X, Y> : A, IB<X, Y> where X : class where Y : class

With these changes in place, your assignment of C into ib is legal.

C c = new C(); 
IB<IEntity, IEntity> ib = c;

(And as Davy8 correctly points out in the comments, this interface covariance feature requires C# 4+.)