The return type for the .NET framework System.Object's method
GetHashCode is a signed integer. However, I've noticed that especially
when doing some XOR magic, the hash code tends to be a large negative
number.

My question is that should user-defined GetHashCode method always return
a nonnegative number? I think that the most common use for hash codes is
something like this:

#  hashTable[obj.GetHashCode() % hashTable.Length] = obj;

But when GetHashCode returns a negative number, the result of the modulo
calculation is negative and therefore the code throws array index exception.