Collection Object - ByRef - ByVal

Question

I am using VBA in Access 2013.

In a regular module there are 2 procedures, RunProc() and PopulateCollection()

When RunProc

Answer 1

In VBA, objects (such as Collections) are always passed by reference. When you pass an object ByRef, the address of the object is passed and PopulateCollection can change the reference.

When you pass it ByVal, a copy of the reference is passed. The copy of the reference still points to the original Collection, but if you change the copy, you don't change the reference in RunProc.

Sub RunProc()

    Dim MyCol As Collection
    Dim i As Long

    Set MyCol = New Collection

    PopCollByVal MyCol

    'I changed what pMyCol points to but *after*
    'I populated it when it still pointed to MyCol
    'so this returns 3
    Debug.Print "ByVal: " & MyCol.Count

    PopCollByRef MyCol

    'When I changed the reference pMyCol it changed
    'MyCol so both became a new Collection. This
    'return 0
    Debug.Print "ByRef: " & MyCol.Count

End Sub

Function PopCollByVal(ByVal pMyCol As Collection)

    Dim i As Long

    'The pointer pMyCol is a copy of the reference
    'to MyCol, but that copy still points to MyCol
    For i = 1 To 3
        'I'm changing the object that is pointed to
        'by both MyCol and pMyCol
        pMyCol.Add "Item" & i
    Next i

    'I can change what pMyCol points to, but I've
    'already populated MyCol because that's what
    'pMyCol pointed to when I populated it.
    Set pMyCol = New Collection

End Function
Function PopCollByRef(ByRef pMyCol As Collection)

    Dim i As Long

    'The pointer pMyCol is the actual reference
    'to MyCol
    For i = 1 To 3
        pMyCol.Add "Item" & i
    Next i

    'When I change what pMyCol points to, I also
    'change what MyCol points to because I passed
    'the pointer ByRef. Now MyCol is a new collection
    Set pMyCol = New Collection

End Function