Question

Consider the following code:

namespace ConsoleApplication1 {
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine(100.CompareTo(200)); // prints -1
            Console.WriteLine(((decimal)100).CompareTo((decimal)200)); // prints -1
            Console.WriteLine(((short)100).CompareTo((short)200)); // prints -100
            Console.WriteLine(((float)100).CompareTo((float)200)); // prints -1
            Console.ReadKey();
        }
    } 
}

My question is, are there any specific reasons the CompareTo-method on Int16 returns values other than -1, 0 and 1?

ILSpy shows it is implemented this way

public int CompareTo(short value)
{
    return (int)(this - value);
}

whereas the method is implented on Int32 this way

public int CompareTo(int value)
{
    if (this < value)
    {
        return -1;
    }
    if (this > value)
    {
        return 1;
    }
    return 0;
}

Answer 1

The difference is that for short, there’s no chance of the result overflowing. For instance, short.MinValue - (short) 1 is still negative – whereas int.MinValue - 1 is int.MaxValue.

In other words, the specific reason is that you can get away with a shortcut with short (no pun intended) whereas the same shortcut doesn’t work with int. You should definitely not require IComparable<T>.CompareTo implementations to return -1, 0 or 1. The documentation is pretty clear that the result is only meaningful in terms of being negative, zero, or positive.