ByRef vs ByVal

This guide discusses VBScript argument passing.

Overview

There has always been confusion about what exactly ByRef and ByVal mean in VBScript. The confusion arises because VBScript uses “by reference” to mean two similar, but different things. VBScript supports:

  1. Reference types
  2. Variable references

The best way to illustrate the difference is with an example. Consider this class:

Class Foo
  Public Bar
End Class

Now we can create an instance of this class:

Dim Blah, Baz
Set Blah = New Foo
Set Baz = Blah
Blah.Bar = 123

Both Blah and Baz are references to the same object. The fourth line changes both Blah.Bar and Baz.Bar because these are different names for the same thing.

That’s the “reference type” feature. We say that VBScript treats objects as reference types.

Now consider this little program:

Sub Change(ByRef XYZ)
  XYZ = 5
End Sub
Dim ABC
ABC = 123
Change ABC

This passes a reference to variable ABC. The local variable XYZ becomes an alias for ABC, so the assignment XYZ = 5 changes ABC as well.