Programming Excel with VBA and .NET

Find Truth

If statements and Do loops rely on Boolean expressions to control what they do. Those Boolean expressions are usually shown as a condition placeholder in the statement’s syntax:

    If condition Then ...

and:

    Do While condition ...

A Boolean expression is simply an item that Visual Basic can determine to be either True or False. Mostly those expressions are very obvious. The fragment If str = "" Then says “if the variable str is an empty string, then execute the following lines of code.” In this case, the equal sign (=) works as a comparison operator, not an assignment operator. Visual Basic can use the operator both ways because it understands that the context of an If statement is different from the standalone statement:

    str = ""

That line performs an assignment, not a comparison! This type of dual use is called overloading . If you hear someone say “operators are overloaded in Visual Basic,” they are just stating that = can be used two different ways.

There’s something else you need to know about Boolean expressions, though. In Visual Basic, any nonzero value is considered to be True. I know that’s weird, but it’s important because it means the following two fragments are equivalent:

    If str = "" Then ...

    If Len(str) Then ...

The second form literally says “if the length of str, then...” which doesn’t make any sense unless you know that 0 equals False and any other value equals True. This second form used to be a common optimization technique because Visual Basic ...