Question

Consider the following two conditionals involving bitwise comparisons in VBScript:

If     1 And 3  Then WScript.Echo "yes" Else WScript.Echo "no"
If Not(1 And 3) Then WScript.Echo "yes" Else WScript.Echo "no"

I think the output should be:

yes
no

But the actual output is:

yes
yes

Wait a second, the Not operator is supposed to perform logical negation on an expression. The logical negation of true is false, as far as I know. Must I conclude that it doesn’t live up to that promise? How and why and what is going on here? What is the rationale, if any?

Answer 1

The VBScript AND operator performs a logical AND operation if both operands are boolean (True, False) — somewhat like the C (style) language && operator.

If both operands are numeric, it will instead perform a bitwise AND operation — somewhat like the C language & operator.

If the operands are of mixed types then the boolean value is cast to a number — False = 0, True = -1 (surprise!) followed by a bitwise AND operation.

So your example evaluates as follows:

'   1 And 3
' = &h0001 And &h0003 <- bitwise AND
' = 1
If 1 Then WScript.Echo "yes" Else WScript.Echo "no" ' Yes

'   Not(1 And 3)
' = Not(1) <- see above
' = Not(&h0001) <- bitwise NOT
' = &hFFFE
If -2 Then WScript.Echo "yes" Else WScript.Echo "no" ' Yes

In case you are wondering how the VBScript NOT operator works, it performs logical negation on a boolean operand like the C language ! operator and bitwise complement on a numeric operand like the C language ~ operator.

In case you want to force logical operations on operands, use the VBScript CBool function to cast the operands:

If Not(CBool(1) And CBool(3)) Then WScript.Echo "yes" Else WScript.Echo "no" ' no

Note: As with most VBScript operators, a Null operand causes the operator to return Null. Null behaves in an unusual way when used inside an If construct.