Question

Why does

var value = new Guid[]{};
var b = (object[])value;

results in

Cannot cast 'value' (which has an actual type of 'System.Guid[]') to 'object[]'

?

UPDATE

Sorry for inaccuracy. The real code is:

    private void SetEditors(object value)
    {
        var array = value as object[];
        ...
    }

and array is null if value is Guid array.

Answer 1

To answer the "why" part- only arrays of reference types are covariant.

So you can write:

string[] x = new string[10];
object[] y = x;

But you can’t write:

int[] x = new int[10];
object[] y = x;

Personally I think it’s a shame that even the first version is allowed, but that’s a different story.

This is all detailed in section 6.1.6 of the C# 4 spec (Implicit Reference Conversions):

The implicit reference conversions are:

• …
• From an array-type S with an element type S_E to an array-type T with an element type T_E provided all of the following are true:
  • S and T differ only in element type. In other words, S and T have the same number of dimensions.
  • Both S_E and T_E are reference-types.
  • An implicit reference conversion exists from S_E to T_E