Question

As my first language and as completely taught from other’s example I never questioned the standard practice in VBA of grouping all variable declarations at the start of the module, routine or function they are scoped to as in this example.

Sub Traditional()
Dim bVariable as Boolean
Dim OtherVariable
' Some code using OtherVariable goes here
'
' Now we use bVariable
bVariable = True
Do While bVariable
    bVariable = SomeFunction()
Loop
End Sub

Now I’m learning that standard practice in other languages is to declare variables as close to where they are used as possible, like this:

Sub Defensive()
Dim OtherVariable as String
' Some code using OtherVariable goes here
'
' Now we use bVariable
Dim bVariable as Boolean
bVariable = True
Do While bVariable
    bVariable = SomeFunction()
Loop
End Sub

This seems completely sensible to me as a defensive programming practice – in that it limits both span and live time (as explained in Code Complete), so I’m wondering if there is any reason for not doing the same in VBA? Possible reasons I can think of are memory, running time (e.g. repeatedly declaring inside a loop), tradition – arguably a good reason as there must be hundreds of thousands of VBA programmers who expect to see all used variables at the start of the routine. Are there any I’ve missed that might explain the benefit of this practice or at least where it came from?

Answer 1

I declare all my variables at the top. I think declaring them closer to first use masks (at least) two other problems that should be fixed first.

1. Procedures too long: If you’re procedure is more than fits on a screen, perhaps it’s doing too much and should be broken into smaller chunks. You’ll also find that unit tests are way easier to write when your procedures are small and only do one thing.
2. Too many variables: If you have a bunch of related variables, consider using a custom class module or user-defined type. It will make the code more readable and easier to maintain.

If your procedures are short and you’re using classes and UDTs, the benefits of declaring the variables at the point of use are lessened or eliminated.