Question

Is there any reason to use if ( object != null ) in a conditional instead of the more concise if ( object ). I see the former more often, but it seems like the two are equivalent and the latter shorter.

Answer 1

The following code will have an object evaluate to false, even though the object is not null:

class SomeWrapper
{
     var value: Object;

     SomeWrapper(Object value)
     {
         this.value = value;
     }

     /// Overriden method from Object, see ActionScript 3 reference
     function valueOf()
     {
         return value;
     }
}

var myWrapper = new SomeWrapper(false);

if(myWrapper)
{
    trace("myWrapper evaluates to true.");
}
else
{
    trace("myWrapper evaluates to false.");
}

The else block will execute in the example above, because myWrapper evaluates to false (its valueOf method returns whatever value the wrapper contained, in this case false) even though myWrapper is not a null reference. The problem is that the above does not only test nullability of reference, it implicitly invokes valueOf method, courtesy of Flash Player virtual machine. This behavior of course may or may not be what you wanted – do you want to test whether myWrapper is a null value or whether it carries a null value? (or both perhaps).

Verbosity in this case is your friend, otherwise you gain code readability in return for potential runtime errors.