For any two reference-types A and B, if an implicit reference conversion (13.1.4) 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: 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[].