Derived List to Base IEnumerable

Question

I have the following code that compiles in .NET Framework version 4.0 and above:

public abstract class MyBase { }
public class MyDerived : MyBase { }

public abs         


        

Answer 1

It's about covariance and contravariance of collections. Check the following link to get more information.

Starting with the .NET Framework 4, several generic interfaces have covariant type parameters; for example: IEnumerable, IEnumerator, IQueryable, and IGrouping. All the type parameters of these interfaces are covariant, so the type parameters are used only for the return types of the members.

Answer 2

In .Net 4.0 the IEnumerable<T> interface was changed from:

public interface IEnumerable<T>

To public interface IEnumerable<out T>

Notice that the word out has been added to the generic type parameter. This means that the generic parameter is co-variant which means you can pass in a more derived type.

Covariance Enables you to use a more derived type than originally specified. You can assign an instance of IEnumerable (IEnumerable(Of Derived) in Visual Basic) to a variable of type IEnumerable

See msdn for more information