List, array and IEnumerable covariance

Question

I'll start with several postulates to better explain the context of my question:

Array Covariance

Postulate 1.1

An array of a value type is not covariant. int[] cannot pass for object[].

Postulate 1.2

An array of a reference type is covariant with a valid IEnumerable. string[] can pass for IEnumerable<object>).

Postulate 1.3

An array of a reference type is covariant with a valid covariant array. string[] can pass for object[].

List Covariance

Postulate 2.1 (same as 1.1)

A list of a value type is not covariant. List<int> cannot pass for List<object>.

Postulate 2.2 (same as 1.2)

A list of a reference type is covariant with a valid IEnumerable. List<string> can pass for IEnumerable<object>).

Postulate 2.3 (different from 1.3)

A list of a reference type is not covariant with a valid covariant List. List<string> cannot pass for List<object>).

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My question concerns postulates 1.3, 2.2 and 2.3. Specifically:

1. Why can string[] pass for object[], but List<string> not for List<object>?
2. Why can List<string> pass for IEnumerable<object> but not for List<object>?

Answer 1

List covariance is unsafe:

List<string> strings = new List<string> { "a", "b", "c" };
List<object> objects = strings;
objects.Add(1);              //

Array covariance is also unsafe for the same reason:

string[] strings = new[] { "a", "b", "c" };
object[] objects = strings;
objects[0] = 1;              //throws ArrayTypeMismatchException

array covariance in C# is recognised as a mistake, and has been present since version 1.

Since the collection cannot be modified through the IEnumerable<T> interface, it is safe to type a List<string> as an IEnumerable<object>.