Why overload true and false instead of defining bool operator?

I've been reading 开发者_如何学Pythonabout 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

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.

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.

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));

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.