#573 – Array Covariance Doesn’t Apply to Value Types

Array covariance allows T[] to be assigned to U[], if T can be assigned to U.

// Assignment compatibility, because Terrier is sub-type of Dog
Terrier t = new Terrier("Bob");
Dog d = t;

// Allowed because of array covariance
Terrier[] terriers = MakeTerrierArray();
Dog[] dogs = terriers;

This does not work, however, if the contents of the arrays are value types.  Arrays of value-typed objects are not covariant.

            byte b1 = 12;
            ushort u1 = b1;  // Assignment compatible

            byte[] bytearray = new byte[] { 1, 2, 3 };

            // Not allowed.  Compile-time error "Cannot implicitly convert type 'byte[]' to 'ushort[]'
            ushort[] shortarray = bytearray;