Covariance and Contravariance in Generics

.NET Framework 4

Covariant and contravariant generic type parameters provide greater flexibility in assigning and using generic types. For example, covariant type parameters enable you to make assignments that look much like ordinary polymorphism. Suppose you have a base class and a derived class, named Base and Derived. Polymorphism enables you to assign an instance of Derived to a variable of type Base. Similarly, because the type parameter of the IEnumerable interface is covariant, you can assign an instance of IEnumerable (IEnumerable(Of Derived) in Visual Basic) to a variable of type IEnumerable, as shown in the following code.