Covariance and Contra-variance

Question

What is covariance and contra-variance in .NET Framework 4.0 with example for each.

Answer 1

In .NET 4.0, CLR supports contra variance and covariance of types in the generic interfaces and delegates.

Covariance allows you to cast the generic type to its base types, that is, instance of type IEnumerable<Tl>  can be assigned 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 permits you to assign a variable of Action<base> to a variable of type Action<derived>. For illustration,
IComparer<object> obj1 = GetComparer()
IComparer<string> obj2 = obj1;

.NET framework 4.0 uses some language keywords (out and in) to annotate contra- variance and covariance. “Out” is used for covariance, while “in” is used for contra-variance.
Variance is applied only to generic interfaces, reference types and generic delegates. These cannot be applied to generic types and value types.