Generic Class Covariance

Question

Is it possible to make the following code compile in C#? I do compile similar in Java.

public interface IInterface
{
    ...
}

public class Class1 : IInterface
{
    ...
}

public abstract class Base<T> where T : IInterface
{
    ...
}

public class Class2<T> : Base<T> where T : IInterface
{
    ...
}

.
.
.

public SomeMethod()
{
    List<Base<IInterface>> list = new List<Base<IInterface>>();
    Class2<Class1> item = new Class2<Class1>();
    list.Add(item); // Compile error here
}

Answer 1

No, that is not legal in C#. C# 4 and above support covariance and contravariance of generic interfaces and generic delegates when they are constructed with reference types. So for example, IEnumerable<T> is covariant, so you could say:

List<Giraffe> giraffes = new List<Giraffe>() { ... };
IEnumerable<Animal> animals = giraffes;

but not

List<Animal> animals = giraffes;

Because a list of animals can have a tiger inserted into it, but a list of giraffes cannot.

Do a web search on covariance and contravariance in C# and you'll find lots of articles on it.

Answer 2

Looks like .NET Framework 4.0 supports covariance in generic interfaces and delegates. So, I happened to compile the code by adding a generic interface.

public interface IInterface
{
    ...
}

public class Class1 : IInterface
{
    ...
}

public interface IBase<out T> where T: IInterface
{
    // Need to add out keyword for covariance.
}

public class Base<T> : IBase<T> where T : IInterface
{
    ...
}

public class Class2<T> : Base<T> where T : IInterface
{
    ...
}

.
.
.

public SomeMethod()
{
    List<IBase<IInterface>> list = new List<IBase<IInterface>>();
    Class2<Class1> item = new Class2<Class1>();
    list.Add(item); // No compile time error here.
}