Developer: RhinoScript
Summary: Discusses VBScript argument passing.

Cannot Use Parentheses When Calling Sub

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.