Why overload true and false instead of defining bool operator?

Question

I've been reading about overloading true and false in C#, and I think I understand the basic difference between this and defining a bool operator. The example I see around is something like:

public static bool operator true(Foo foo) {
  return (foo.PropA > 0);
}
public static bool operator false(Foo foo) {
  return (foo.PropA <= 0);
}

To me, this is the same as saying:

public static implicit operator bool(Foo foo) {
  return (foo.PropA > 0);
}

The difference, as far as I can tell, is that by defining true and false separately, you can have an object that is both true and false, or neither true nor false:

public static bool operator true(Foo foo) { return true; }
public static bool operator false(Foo foo) { return true; }
//or
public static bool operator true(Foo foo) { return false; }
public static bool operator false(Foo foo) { return false; }

I'm sure there's a reason this is allowed, but I just can't think of what it is. To me, if you want an object to be able to be converted to true or false, a single bool operator makes the most sense.

Can anyone give me a scenario where it makes sense to do it the other way?

Thanks

I've just found these operators as well and did some tests. It seems like `if` only uses the `true` operator, and `if (!myObj)` is not possible at all, so there's no point (yet) in implementing a contradictory `false` operator. Did I miss anything? How can I use the `false` operator at all? — ygoe, Sep 14 '13 at 12:31

score 13 · Accepted Answer · answered Apr 19 '10 at 21:42

13

As the docs say, overloading true and false is intended to support (nullable) database-types (Yes/No, Y/N, 0/1, etc).

And of course you can define them inconsistently, as with any operator. It is your responsibility to return something sensible. The compiler goes no further than requiring neither or both.

answered Apr 19 '10 at 21:42

H H

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Thanks - I didn't get that far in MSDN - I started looking elsewhere after their [operator overloading](http://msdn.microsoft.com/en-us/library/aa288467(VS.71%29.aspx "operator overloading") page, which wasn't much help. I suppose I didn't think about the fact that you can overload ==/!= the same way, where they conflict with each other, I just could never imagine actually doing it. Thanks for the tip. – Joe Enos Apr 19 '10 at 22:04

score 8 · Answer 2 · answered Apr 20 '10 at 01:53

I had no idea these operators existed. That means you can implement the self-negation paradox:

public class ThisClassIsFalse
{
    public static bool operator true(ThisClassIsFalse statement)
    {
        return statement ? false : true;
    }

    public static bool operator false(ThisClassIsFalse statement)
    {
        return statement ? true : false;
    }
}

So now we know the true solution to this classic paradox... StackOverflowException.

public bool ThisMethodReturnsFalse() { return true; } – Joe Enos Apr 20 '10 at 04:20 — Joe Enos, Apr 20 '10 at 04:20

score 2 · Answer 3 · answered Apr 19 '10 at 21:58

2

I've seen people overload the true and false overloads in order to do clever things like building expressions in .NET 2.0, before Linq existed.

Ayende worked out a syntax like this to build NHibernate criteria queries, using his NHQG project:

return Repository.FindAll(
    (Where.Publisher.Name == name) &&
    (Where.Publisher.City == city));

answered Apr 19 '10 at 21:58

Fábio Batista

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5

Ahh, the days before LINQ...Those were innocent times when people did atrocious things that were clever at the time... – Joe Enos Apr 19 '10 at 22:08

score 1 · Answer 4 · answered Apr 19 '10 at 21:29

1

Depending on the system, true can be any non-zero value. In others, it can be any positive value.

Other systems aren't truly boolean, and allow a third state null or nill for the boolean values, which is why you might overload true and false, versus overloading a single bool operator.

answered Apr 19 '10 at 21:29

Benjamin Anderson

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This is specifically for .NET, so are you saying that it's for .NET applications talking to non-.NET code? – Joe Enos Apr 19 '10 at 21:35
Joe, this is a feature in the specification for C#, not .NET. C# could be used in any system, and true and false might need their definition changed. – Benjamin Anderson Apr 20 '10 at 14:18
Benjamin: Thanks for the response - a few comments: First, I believe this is the case for other .NET languages, not just C# - VB.NET has special operators IsTrue and IsFalse which seem to be the same thing, unless I'm mistaken. Also, you mentioned that C# could be used in any system - I've heard a few people say things like this in the past, but I've never heard explanations. Other than Mono, is there any other place outside of .NET where C# is actually used? Thanks – Joe Enos Apr 20 '10 at 15:45
Mono is the primary alternative implementation of C#, but C# the language is an open ISO standard, and can be used and implemented by anyone that wants to do the work to implement it. – Benjamin Anderson Apr 20 '10 at 20:25

Why overload true and false instead of defining bool operator?

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