Question

Quite simply, why does this code fail to compile?

public interface IWorld { }
public class Foo<T> where T : IWorld { }
public void Hello<T>(T t) where T : IWorld
{
    Foo<IWorld> bar1 = new Foo<T>(); //fails implicit cast
    Foo<IWorld> bar2 = (Foo<IWorld>)new Foo<T>(); //fails explicit cast
}

Since every T implements IWorld, every instance of Foo<T> should match Foo<IWorld>. Why not? Is there any way around this? I really don’t want to resort to generics to accomplish this.

Answer 1

T : IWorld

means that T has been implemented IWorld and does not mean that it ONLY has implemented IWorld and EXACTLY is IWorld. It may also has been implemented other interfaces.

However, C# supports this cast in it’s later versions. Please see http://msdn.microsoft.com/en-us/library/dd799517.aspx (Covariance and Contravariance in Generics)