In .NET 4.0, the CLR supports covariance and contravariance of types in generic interfaces and delegates. Covariance enables you to cast a generic type to its base types, that is, you can assign a instance of type IEnumerable<Tl> to a variable of type IEnumerable<T2> where, T1 derives from T2. For example,
IEnumerable<string> str1= new List<string> ();
IEnumerable<object> str2= str1;
Contravariance allows you to assign a variable of Action<base> to a variable of type Action<derived>. For example,
IComparer<object> obj1 = GetComparer()
IComparer<string> obj2 = obj1;
.NET framework 4.0 uses some language keywords (out and in) to annotate covariance and contra-variance. Out is used for covariance, while in is used for contra-variance.
Variance can be applied only to reference types, generic interfaces, and generic delegates. These cannot be applied to value types and generic types.