In C#/.NET why is sbyte[] the same as byte[] except that it's not?

Question

I just observed a weird phenomenon in C#/.NET.

I created this minimal example to demonstrate:

if (new sbyte[5] is byte[])
{
 throw new ApplicationExc         


        

Answer 1

Here's a simpler example that shows the same issue:

static void Main(string[] args)
{
    bool a = ((object) new byte[0]) is sbyte[];
    bool b = (new byte[0]) is sbyte[];

    Console.WriteLine(a == b); // False
}

The inconsistency arises because the C# compiler decides that it knows the result of (new byte[0]) is sbyte[] at compile time, and just substitutes false. Perhaps it should really substitute true, to be more consistent with the CLR behaviour.

As far as I can tell, it's only this little optimization that's inconsistent. It occurs only when both sides of the is expression are statically typed as an array whose element type is a signed or unsigned integer or an enum, and the sizes of the integers are the same.

The good news is that while this might seem inconsistent, C# will always issue a warning when it substitutes false into such expressions – in practice, I think this might be more useful than quietly returning true.