Question

If the Covariant type parameters which added in .Net Framework 4 enable me to make assignments that look like polymorphism

Why I can write

IEnumerable<object> lst = new List<string>();

And I can’t write

List<object> lst = new List<string>();

Answer 1

There are 2 reasons why it doesn’t work:

• IEnumerable<T> is covariant (it is declared public interface IEnumerable<out T>), but IList<T> isn’t. Otherwise you would be able to do something like that:

  IList<string> listOfStrings = ...
  IList<object> listOfObjects = listOfStrings;
  listOfObjects.Add(42); // would fail at runtime because it is actually a list of strings, not ints
  

• Covariance and contravariance only work on interfaces, not on concrete classes

From MSDN:

In the .NET Framework version 4, variant type parameters are restricted to generic interface and generic delegate types.