Is it possible to simulate tri-state checkboxes in Microsoft TreeView Control 6.0 (MSComctlLib.TreeCtrl.2)?

I'm using the Microsoft TreeView Control 6.0 in Microsoft Access. It seems to work very well, except that it doesn't seem to have a greyed out state, indicating that some, but not all child nodes are checked.

I've looked into using my own images to simulate the checkboxes, but if I do this, I then have to remove the real checkboxes or it looks like I have two checkboxes for each item... but then I don't have any checkboxes and I can't work out how to handle a click on the images.

I can find loads of people having the same sort of questions for this control in other languages/uses, but I can't find a solution for Microsoft Access.

I would happy moving over to a different control, if there's something else available that gives me a hierarchical structure with tri-state checkboxes.

After a bit of research and a couple of hours coding, I was able to write a solution myself.

I had to add an ImageList, associate that to the TreeView and add an image of a checkbox for each of the three states. Google Image search saved me some time here :).

'Enumeration for simulated tri-state checkboxes, matching up to the TreeView's associated Image List's Index
Private Enum CheckboxEnum
  Unchecked = 1
  Checked = 2
  Partial = 3
End Enum

'---------------------------------------------------------------------------------------
' Procedure : objTreeView_MouseDown
' Author    : Matty Brown
' Date      : 19/05/2014
' Purpose   : Because TreeView doesn't support tri-state checkboxes, these have to be simulated using images.
'---------------------------------------------------------------------------------------
'
Private Sub objTreeView_MouseDown(ByVal Button As Integer, ByVal Shift As Integer, ByVal x As stdole.OLE_XPOS_PIXELS, ByVal y As stdole.OLE_YPOS_PIXELS)
  Const CHECKBOX_WIDTH As Integer = 195 '195=13px

  Dim objNode As Node
  Set objNode = objTreeView.HitTest(x, y)

  If objNode Is Nothing Then
    'Miss
  Else
    'Find left side of node by moving left one pixel at a time until you fall off the node, then move one pixel to the right
    Dim intX As stdole.OLE_XPOS_PIXELS
    For intX = x To 0 Step -15
      If Not objNode Is objTreeView.HitTest(intX, y) Then
        If x <= intX + CHECKBOX_WIDTH Then
          'User clicked on the checkbox
          Select Case objNode.Image
            Case CheckboxEnum.Unchecked:
              objNode.Image = CheckboxEnum.Checked
            Case Else:
              objNode.Image = CheckboxEnum.Unchecked
          End Select

          'Recursively check child nodes
          Call CheckTreeNodes(objTreeView, objNode, objNode.Image)

          'Update parent node(s)
          Call UpdateParentNodes(objTreeView, objNode)
        Else
          'User clicked outside of the checkbox
          '
        End If

        Exit For
      End If
    Next
  End If
End Sub

'---------------------------------------------------------------------------------------
' Procedure : CheckTreeNodes
' Author    : Matty Brown
' Date      : 16/05/2014
' Purpose   : Checks or unchecks all of the child nodes for the specified node
'---------------------------------------------------------------------------------------
'
Private Sub CheckTreeNodes(ByRef tv As TreeView, ByRef nodNode As Node, ByVal Value As CheckboxEnum)
  Dim lngIndex As Long

  'Cascade change to children
  If nodNode.Children > 0 Then
    lngIndex = nodNode.Child.Index
    Call CheckTreeNodes(tv, tv.Nodes(lngIndex), Value)

    Do While lngIndex <> nodNode.Child.LastSibling.Index
      lngIndex = tv.Nodes(lngIndex).Next.Index
      Call CheckTreeNodes(tv, tv.Nodes(lngIndex), Value)
    Loop
  End If

  nodNode.Image = Value
End Sub

'---------------------------------------------------------------------------------------
' Procedure : CountChildNodes
' Author    : Matty Brown
' Date      : 19/05/2014
' Purpose   : Counts how many child nodes are checked or unchecked, so that a parent node can be set correctly
'---------------------------------------------------------------------------------------
'
Private Sub CountChildNodes(ByRef tv As TreeView, ByRef nodNode As Node, ByRef lngChecked As Long, ByRef lngUnchecked As Long)
  Dim lngIndex As Long

  'Check this node's children
  If nodNode.Children > 0 Then
    lngIndex = nodNode.Child.Index
    Call CountChildNodes(tv, tv.Nodes(lngIndex), lngChecked, lngUnchecked)

    Do While lngIndex <> nodNode.Child.LastSibling.Index
      lngIndex = tv.Nodes(lngIndex).Next.Index
      Call CountChildNodes(tv, tv.Nodes(lngIndex), lngChecked, lngUnchecked)
    Loop
  Else
  'Update totals
    Select Case nodNode.Image
      Case CheckboxEnum.Checked:
        lngChecked = lngChecked + 1
      Case CheckboxEnum.Unchecked:
        lngUnchecked = lngUnchecked + 1
    End Select
  End If
End Sub


'---------------------------------------------------------------------------------------
' Procedure : UpdateParentNodes
' Author    : Matty Brown
' Date      : 19/05/2014
' Purpose   : Steps through parent nodes, updating them according to how many checked/unchecked child nodes they have
'---------------------------------------------------------------------------------------
'
Private Sub UpdateParentNodes(ByRef tv As TreeView, ByRef nodNode As Node)
  Dim lngIndex As Long
  Dim nodParent As Node
  Dim lngChecked As Long, lngUnchecked As Long

  'If this node has no parents, there's nothing to update
  If nodNode.Parent Is Nothing Then Exit Sub
  Set nodParent = nodNode

  Do While Not nodParent.Parent Is Nothing
    Set nodParent = nodParent.Parent

    'Reset counters
    lngUnchecked = 0
    lngChecked = 0

    'Count children
    Call CountChildNodes(tv, nodParent, lngChecked, lngUnchecked)

    'Update parent nodes
    If lngUnchecked = 0 And lngChecked > 0 Then
      nodParent.Image = CheckboxEnum.Checked
    ElseIf lngUnchecked > 0 And lngChecked > 0 Then
      nodParent.Image = CheckboxEnum.Partial
    Else
      nodParent.Image = CheckboxEnum.Unchecked
    End If
  Loop
End Sub