19.5 Array covariance

For any two reference-types A and B, if an implicit reference conversion (§1
3.1.4) or explicit reference
conversion (§13.2.3) exists from A to B, then the same reference conversion
also exists from the array type
A[R] to the array type B[R], where R is any given rank-specifier (but the
same for both array types). This
relationship is known as array covariance. Array covariance, in particular,
means that a value of an array
type A[R] may actually be a reference to an instance of an array type B[R],
provided an implicit reference
conversion exists from B to A.
Because of array covariance, assignments to elements of reference type
arrays include a run-time check
which ensures that the value being assigned to the array element is
actually of a permitted type (§14.13.1).
[Example: For example:
class Test
{
static void Fill(object[] array, int index, int count, object value) {
for (int i = index; i < index + count; i++) array[i] = value;
}
static void Main() {
string[] strings = new string[100];
Fill(strings, 0, 100, "Undefined");
Fill(strings, 0, 10, null);
Fill(strings, 90, 10, 0);
}
}
Chapter 19 Arrays
277
The assignment to array[i] in the Fill method implicitly includes a
run-time check, which ensures that
the object referenced by value is either null or an instance of a type that
is compatible with the actual
element type of array. In Main, the first two invocations of Fill succeed,
but the third invocation causes a
System.ArrayTypeMismatchException to be thrown upon executing the first
assignment to
array[i]. The exception occurs because a boxed int cannot be stored in a
string array. end example]
Array covariance specifically does not extend to arrays of value-types. For
example, no conversion exists
that permits an int[] to be treated as an object[].