Question

I need to allow the vertical scrollbar in a multiselect listbox (VB6) however, when the control is disabled, I can’t scroll.

I would think there is an API to allow this, but my favorite VB6 site (MVPS VB.NET) does not have a way.

I toyed with pretending it was disabled, and ignore the clicks… but to do that with VB6 code is really ugly… so if this is a solution, I need an API to ignore the clicks.

Thanks for your help.

Answer 1

I came up with the following code, which hides all of the gnarly details behind a class. Basically, I implemented greg’s idea of using overlaying another scrollbar on top of the disabled list box’s scrollbar. In my code, I dynamically create another ListBox control (resized so that only its scrollbar is visible), and use its scrollbar to scroll the actual ListBox. I also specifically avoided using the Windows API (except for the call to GetSystemMetrics that I used to figure how how wide a scroll bar is on the system). The nice thing about using another ListBox’s scrollbar is that it will be themed properly (a ListBox uses the OS’s theme when it displays it’s scrollbar, but a VB.Scrollbar doesn’t, so it would look out-of-place). Another advantage of using a second ListBox to scroll the first list box is that it’s really easy to implement the scrolling logic (just set the first ListBox’s TopIndex property to the second ListBox’s TopIndex property whenever the second one is scrolled).

I also set it up to be as low-impact as possible (you only have to call a single function in your Form_Load event to make it work).

Usage

1. Add CustomScrollingSupport.cls and ListBoxExtras.bas to your project.

2. In your form’s Form_Load event, add the following line:

   AddCustomListBoxScrolling Me

   This will make every VB.ListBox on the form support scrolling even while they are disabled. If you only want to add this functionality to a select number of ListBox’s, you can call AddCustomScrollingSupport instead, passing in a specific ListBox control.

Interesting Note

In an older version of this code, I wasn’t calling the ZOrder method on the second listbox (the one that provides the scrollbar) to make sure it would appear on top of the first listbox. This meant the second listbox was actually behind the first listbox; the interesting thing is that the scrolling on the second ListBox still worked when the first ListBox was disabled! Apparently, when the first ListBox is disabled, any mouse and keyboard events that would have gone to that ListBox ‘bleed through’ to the second ListBox, so scrolling support still does work. I’m not sure if this is a bug or by design (I mean, you could argue that it makes sense that controls behind a disabled control would be able to receive events…). However, I found the scrolling to be slightly jerky at times, so I decided to add .ZOrder 0 to make the second listbox render on top of the first one. This has the drawback that you see the frame border for the second listbox (to the left of the scroll bar), which you wouldn’t see if it was hidden behind the first listbox, but the scrolling is smoother.

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CustomScrollingSupport.cls

This class wraps up the logic necessary to add ‘custom scrolling support’ (for lack of a better name) to a VB.ListBox control. It should not be used directly, instead use the one of the Add* methods in the ListBoxExtras.bas module (I’ll provide the code for that module later in the post).

Option Explicit  Private Declare Function GetSystemMetrics Lib 'user32' (ByVal nIndex As Long) As Long Private Const SM_CXVSCROLL = 2 Private Const SM_CXFRAME = 32  Private m_runningScrollers As Collection Private WithEvents m_list As VB.listbox Private WithEvents m_listScroller As VB.listbox  '--------------------------------------------------------------' ' Bind                                                         ' '                                                              ' '   Adds custom scrolling support to a ListBox control.        ' '   Specifically, it allows the ListBox to be                  ' '   scrolled even when it is disabled.                         ' '                                                              ' '   Parameters:                                                ' '                                                              ' '   + list                                                     ' '       the ListBox control to add custom scrolling support to ' '                                                              ' '   + runningScrollers                                         ' '       a Collection of CustomScrollingSupport objects. Passed ' '       in so that this object can remove itself from the list ' '       when it is terminated.                                 ' '                                                              ' '--------------------------------------------------------------'  Public Sub Bind(ByVal list As VB.listbox, runningScrollers As Collection)      Set m_list = list     Set m_runningScrollers = runningScrollers      'Create another ListBox loaded with the same number of entries as the real listbox'     Set m_listScroller = m_list.Container.Controls.Add('VB.ListBox', list.Name & '_scroller')     LoadScrollerList      Dim nScrollbarWidth As Long     nScrollbarWidth = GetSystemMetricScaled(SM_CXVSCROLL, m_list) + _                       GetSystemMetricScaled(SM_CXFRAME, m_list)      'Display the other listbox (the 'scroller'), just wide enough so that only its scrollbar is visible'     'and place it over the real listboxs scroll bar'     With m_listScroller         .Left = m_list.Left + m_list.Width - nScrollbarWidth         .Top = m_list.Top         .Height = m_list.Height         .Width = nScrollbarWidth         .Enabled = True         .Visible = True         .ZOrder 0     End With  End Sub  Private Sub m_listScroller_Scroll()     'If the master list has changed, need to reload scrollers list'     '(not ideal, but there is no ItemAdded event that we could use to keep the lists in sync)'     If m_list.ListCount <> m_listScroller.ListCount Then         LoadScrollerList     End If      'Make any scrolling done on the scroller listbox occur in the real listbox'     m_list.TopIndex = m_listScroller.TopIndex  End Sub  Private Sub Class_Terminate()      Dim scroller As CustomScrollingSupport     Dim nCurrIndex As Long      If m_runningScrollers Is Nothing Then         Exit Sub     End If      'Remove ourselves from the list of running scrollers'      For Each scroller In m_runningScrollers         nCurrIndex = nCurrIndex + 1         If scroller Is Me Then             m_runningScrollers.Remove nCurrIndex             Debug.Print m_runningScrollers.Count & ' scrollers are running'             Exit Sub         End If     Next  End Sub  Private Sub LoadScrollerList()      Dim i As Long      m_listScroller.Clear     For i = 1 To m_list.ListCount         m_listScroller.AddItem ''     Next  End Sub  Private Function GetSystemMetricScaled(ByVal nIndex As Long, ByVal ctrl As Control)     GetSystemMetricScaled = ctrl.Container.ScaleX(GetSystemMetrics(nIndex), vbPixels, ctrl.Container.ScaleMode) End Function 

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ListBoxExtras.bas

This module contains two utility methods:

AddCustomScrollingSupport adds custom scrolling functionality to an individual VB.ListBox control

AddCustomListBoxScrolling adds custom scrolling functionality to every VB.ListBox control on a given Form

Option Explicit  Public Sub AddCustomScrollingSupport(ByVal list As VB.listbox)      Static runningScrollers As New Collection      Dim newScroller As CustomScrollingSupport     Set newScroller = New CustomScrollingSupport      runningScrollers.Add newScroller     newScroller.Bind list, runningScrollers  End Sub  Public Sub AddCustomListBoxScrolling(ByVal frm As Form)      Dim ctrl As Control     For Each ctrl In frm.Controls          If TypeOf ctrl Is VB.listbox Then             AddCustomScrollingSupport ctrl         End If      Next  End Sub