Why is comparing a struct to NULL legal in C#?

Question

Possible Duplicate:
C# okay with comparing value types to null

Consider the following code with the TimeSpan, which is a struct:

// will not compile - illegal
TimeSpan ts = null;

However, the following code does compile and is legal, though the expression is always false:

if (ts == null)
    Console.WriteLine("this line will never be hit");

Can someone tell me why it's invalid to set a struct to a NULL, but it's ok to compare it to one?

http://stackoverflow.com/questions/2022425/comparing-structs-to-null, http://stackoverflow.com/questions/648115/how-can-an-object-not-be-compared-to-null, http://stackoverflow.com/questions/1225949/why-can-timespan-and-guid-structs-be-compared-to-null, http://stackoverflow.com/questions/1972262/c-sharp-okay-with-comparing-value-types-to-null — nawfal, Apr 09 '12 at 23:44
If you couldn't compare structs to null, what would you do with Nullable? — Anthony Pegram, Apr 09 '12 at 23:46
@AnthonyPegram: Better question is; how would you implement `Nullable` as a `struct`? (of course, you couldn't). `Nullable` could of course be implemented as a reference type — Ed S., Apr 10 '12 at 00:02

score 7 · Answer 1 · answered Apr 09 '12 at 23:43

7

It's still legal because you can overload the == operator for structs.

struct AmNull {
    public static bool operator ==(AmNull a, object b) {
        return b == null;
    }

    public static bool operator !=(AmNull a, object b) {
        return b != null;
    }
}

...

Console.WriteLine(new AmNull() == null); // True

answered Apr 09 '12 at 23:43

Ry-

218,210
55
464
476

3

@AngrHacker: I think you should chose this response as the accepted answer. It covers my response but expands upon it to explain why this behavior is present. – Ed S. Apr 10 '12 at 17:00

score 3 · Accepted Answer · answered Apr 09 '12 at 23:42

3

Fails to compile for me:

struct Foo { }

class Program
{       
    static void Main( string[] args )
    {
        var f = new Foo();
        if( f == null ) { }
    }
}

Error 1 Operator '==' cannot be applied to operands of type 'ConsoleApplication3.Foo' and 'null'

answered Apr 09 '12 at 23:42

Ed S.

122,712
22
185
265

@AngryHacker: It's because `TimeSpan` overloads the `==` operator (I didn't catch that at first). My point is that your question "Why is comparing a struct to NULL legal in C#?" isn't complete; you can't, unless you overload the equality operator to accept it. minitech has a better response. – Ed S. Apr 09 '12 at 23:45

Why is comparing a struct to NULL legal in C#?

2 Answers2