Why don't covariant type parameters allow me to write [List <object> lst = new List <string> ()]?

Question

If the Covariant parameters that are added in .Net Framework 4 allow me to make assignments that look like polymorphism

Why can I write

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

And I can not write

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

Answer 1

There are two reasons why this does not work:

• IEnumerable<T>is covariant (declared public interface IEnumerable<out T>), but IList<T>not. Otherwise, you can do something like this:

  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 specific classes.

From MSDN :

.NET Framework 4 .

Answer 2

List {T} , :

List<object> lst = new List<string>();
lst.Add(10);

, Int32 .

IEnumerable {out T} , 'out'. , , , .

, , :

List<object> lst = new List<object>();
lst.AddRange(new string[] { "hello" });