Question

When is a Dim statement processed in the compiling of VBA code? Is there any efficiency gain if I do this?:

Dim oFileDiag As FileDialog

Set oFileDiag = Application.FileDialog(msoFileDialogFilePicker)                
If oFileDiag.Show = -1 Then

    '// Dim statement further down in the code...
    Dim ofdSelected As FileDialogSelectedItems
    Set ofdSelected = .SelectedItems

End If

As opposed to this?:

'// Dim statement at the beginning of the code...
Dim oFileDiag As FileDialog
Dim ofdSelected As FileDialogSelectedItems

Set oFileDiag = Application.FileDialog(msoFileDialogFilePicker)                
If oFileDiag.Show = -1 Then

    Set ofdSelected = .SelectedItems

End If

Answer 1

As I understand it (I’ve been wrong before), there is no gain in efficiency in VB(A). Declaring any variable anywhere in the routine will use the same resources (though your variable won’t be available until below its declaration).

The declaration of an object variable (Dim) only creates the reference, it does not instantiate the object until the Set. However, watch out for the opposite problem in this sort of construct:

Dim rsFoo As New ADODB.Recordset

'other statements

With rsFoo
    .LockType = adBatchOptimistic
    'other statements
End With

This seems like it might be convenient, since you don’t have to do an explicit Set... = New.... But the gotcha is that every time you use rsFoo at runtime the code has to check whether or not it’s been instantiated. Much better this way:

Dim rsFoo As ADODB.Recordset

'other statements

Set rsFoo = New ADODB.Recordset
With rsFoo
    .LockType = adBatchOptimistic
    'other statements
End With

This is, to some extent, a “religious” difference, at least in languages where there isn’t any real difference in overhead. The most common practice in VB(A) is to declare all variables at the beginning of the routine, though there are some who argue that keeping the declaration as close as possible to the first use is “clearer” (not to those of us who are conditioned to expect them all at the beginning, it’s not…).