VBS logical operators initialise empty variables?

Question

Consider the following bit of VBS:

dim msg, myVar, myVar2

msg = "myVar = " & myVar
msg = msg & vbcrlf & "myVar = empty: " & isempty(myVar)
msg = msg & vbcrlf & "TypeName(myVar) = " & TypeName(myVar)

msgbox msg, , "BEFORE"

if not myVar then myVar2 = true

msg = "myVar = " & myVar
msg = msg & vbcrlf & "myVar = empty: " & isempty(myVar)
msg = msg & vbcrlf & "TypeName(myVar) = " & TypeName(myVar)

msgbox msg, , "AFTER"

I would expect the output from "BEFORE" and "AFTER" to be the same... all we're doing is making a comparison to an uninitialised (empty) variant right?

However - it seems like the "if not" actually initialises it to a (long)zero! I've been coding in VBS (ASP) for donkey's years, and this is a new one on me!

A few things to note:

• The behaviour is the same in both a .vbs and the equivalent ASP code (on my Win 7 desktop and on Server 2008 R2.)
• All logical operators - and/or/not/xor produce this effect
• Comparison operators do not.

It seems like a potential trap for the unwary... Can anyone explain this behaviour?

Answer 1

I couldn't find anything official about this issue. After doing some test, I decided that this is a bug or an un-documented effect. This behaviour does not apply on other similar platforms like VBA and VB6.

Visual Basic for Application:

Visual Basic 6:

VBScript:

As a workaround, passing expressions by value works.

If Not (exp) Then
'or
If Not CBool(exp) Then

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ByRef and ByVal Parameters
CBool Function