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12.5 Array covariance

Visual Studio .NET 2003

For any two reference-types A and B, if an implicit reference conversion (Section 6.1.4) or explicit reference conversion (Section 6.2.3) 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 that ensures that the value being assigned to the array element is actually of a permitted type (Section 7.13.1). 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);
   }
}

The assignment to array[i] in the Fill method implicitly includes a run-time check that 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.

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[].

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