Re: Comparison to null

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From: Jon Skeet [C# MVP] (skeet_at_pobox.com)
Date: 03/15/05 

Date: Tue, 15 Mar 2005 17:51:14 -0000

Marcin Grz?bski <mgrzebski@taxussi.no.com.spam.pl> wrote:
> I think that a "key" is operator overloading.
> Like a Jon post it before.
>
> MyClass myVar;
>
> if( null==myVar ) {
> }
> Doesn't have a chance to fire MyClass "==" operator function.

Yes it does. Compile and run the following, ignoring the warnings:

using System;
using System.Data;

class Test
{
    static void Main()
    {
        Test t = new Test();
        Console.WriteLine (t==null);
        Console.WriteLine (null==t);
    }

    public static bool operator == (Test t1, Test t2)
    {
        Console.WriteLine ("Operator== called");
        Console.WriteLine ("t1={0}", t1);
        Console.WriteLine ("t2={0}", t2);
        return true;
    }
    
    public static bool operator!= (Test t1, Test t2)
    {
        return false;
    }
}

The output is:
Operator== called
t1=Test
t2=
True
Operator== called
t1=
t2=Test
True

In other words, the overloaded operator is called both times. 

-- 
Jon Skeet - <skeet@pobox.com>
http://www.pobox.com/~skeet
If replying to the group, please do not mail me too

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