Friday, June 15, 2007

Overloading == to return a non-boolean

by skeet via Jon Skeet's Coding Blog : C# on 6/15/2007 5:38:42 AM

Were you aware that you could overload == to return types other than boolean? I certainly wasn't until I started reading through the lifted operators part of the C# 2 specification. It's quite bizarre - here it is in action:

using System;

class Test
{
    public static string operator== (Test t1, Test t2)
    {
        return "Fish?";
    }
    
    public static string operator!= (Test t1, Test t2)
    {
        return "Not a fish?";
    }

    static void Main()
    {
        Test a = new Test();
        Test b = new Test();
        Console.WriteLine (a==b);
    }
}

That ends up printing "Fish?" to the console. Strange but true. When I asked about this on the newsgroup, one poster said that he did use this functionality for an ORM type of system - his == operator on two expressions would return another expression which represented the test for equality between the other two expressions. I can't say I much like this idea, although I could see where he was coming from. (The C# spec does specifically discourage this sort of thing, which is at least a start).

So, dear readers, have any of you done this, and if so, why?