VBScript Logic
Windows only

## Logic?

Consider the following statements:

If blnResult = True Then Print "True!" Else Print "False!"


and

If blnResult Then Print "True!" Else Print "False!"


Is there a difference?

## What Logic?

Yes, there is a big difference. If blnResult is True or False, then both statements do what you would expect – the same thing. But, the first statement is asking “Is blnResult equal to True?” whereas the second question is asking “Is blnResult not equal to False?”

In a strictly Boolean world, those are equal statements. But the VBScript type system is richer than just Booleans.

## Details

For example, what if - in the above example - blnResult is the string True? The string True is not equal to the Boolean True, so the first statement is false. But the string is also not equal to False, so the second statement is true, and the statements have different semantics.

The same goes for numbers. When converted to a number, True converts to -1 (for reasons which will become clear in a moment) and False converts to 0. So, if blnResult is 1, again the first statement is false because 1 <> -1, and the second statement is true because 1 <> 0.

What’s going on is that VBScript is not logical. VBScript is bitwise. All the so-called logical operators work on numbers, not on Boolean values. Not, And, Or, XOr, Eqv and Imp all convert their arguments to four-byte integers, do the logical operation on each pair of bits in the integers, and return the result. If True is -1 and False is 0 then everything works out, because -1 has all its bits turned on and 0 has all its bits turned off. But if other numbers get in there, all bets are off.

This can lead to some strange situations if you’re not careful. In VBScript, it is certainly possible for…

If blnResult Then


and

If blnAnswer Then


to be both true, but

If Blah And Foo Then


to be false, if blnResult is 1 and blnAnswer is 2, for example.

## Best Practices

Conditional statements should always take Booleans. Or, in other words, use Booleans as Booleans. Use nothing else as Booleans.

Suppose you’ve got a method that returns a number and you want to do something if it doesn’t return zero. Don’t do this, even though it does exactly what you want:

If intResult Then


it’s clearer to call it out and make the conditional take a Boolean:

If intResult <> 0 Then


Conversely, if a value is a Boolean and you know that, there’s no need to compare it. When you see:

If blnResult = True Then


If blnResult can only contain True or False, then you can just say

If Blah Then


Use the same practice with logical operators. Do not mix-and-match. Either every argument should explicitly be a number and you’re doing bitwise comparisons, or every argument is a Boolean. Mixing the two makes the code harder to read and more bug-prone.