cListboxHScroll class

A helper class that adds a functional horizontal scrollbar to a standard VB ListBox control by combining the LB_SETHORIZONTALEXTENT message, Win32 scroll-info APIs, and window subclassing.

The class measures the pixel width of each list item using GDI’s GetTextExtentPoint32W, sets the scroll range to fit the widest item, and installs a subclass procedure to intercept WM_HSCROLL messages and set the scrollbar thumb’s position. Call Show after populating the list and UnShow (or allow the class instance to be destroyed) when the scroll handling is no longer needed.

Private HScroll As New cListboxHScroll

Private Sub Form_Load()
    List1.AddItem "A short item"
    List1.AddItem "A much longer item that overflows the default width"
    HScroll.Show List1
End Sub

Private Sub Form_Unload(Cancel As Integer)
    HScroll.UnShow
End Sub

Methods
Example
Remarks
See Also

Methods

Show

Activates horizontal scrolling on a ListBox control.

Syntax: object.Show ListBoxInst

ListBoxInst: required An Object reference to the ListBox control to instrument. The class assigns it to a ListBox variable, so an object of another type causes a run-time error. The control must already have its items populated; Show measures the text width of every item at call time to compute the scroll range.

Show performs three operations in sequence:

Calls the Win32 ShowScrollBar API to make the horizontal scrollbar visible.
Iterates every list item, measures its pixel width using the GDI device context of the control, and sends LB_SETHORIZONTALEXTENT with the widest measurement plus a 10-pixel margin. It also calls SetScrollInfo to set the scroll range and page size so the scrollbar thumb reflects the visible proportion of the content. When the list is empty, or every item is empty, neither is set.
Installs a window subclass procedure via SetWindowSubclass (comctl32) to intercept WM_HSCROLL messages. The subclass updates the scroll position on line-left / line-right (20 px per step), page-left / page-right, thumb-track / thumb-position, and left / right scrolls, then passes the message to DefSubclassProc.

The scroll range is calculated from the widest item’s pixel width measured with the font that the control reports through WM_GETFONT, so a ListBox that uses a non-default font is measured with that font.

Warning 1.0.0.1

The thumb of the scroll bar and the contents of the list disagree after a click on a scroll arrow or in the track of the scroll bar. The subclass moves the thumb by 20 pixels for an arrow and by one page for the track, while the list box scrolls its contents by its own step, for example 9 pixels for an arrow and about two-thirds of a page for the track. After a few clicks the thumb is at a position the list does not show. Dragging the thumb, and the scroll to the start or the end, leave them equal.

Note

Show measures item widths at the moment it is called. If items are added or removed after Show, the scroll range is not updated automatically. Call Show again after bulk changes to refresh the extent.

Important

The control’s hWnd must be valid (the form must be loaded and the control fully created) before calling Show. Calling Show before the control’s window exists returns silently without installing any scrolling behavior.

UnShow

Removes the window subclass installed by Show.

Syntax: object.UnShow

Calls RemoveWindowSubclass to detach the WM_HSCROLL handler. After UnShow the ListBox no longer receives the class’s WM_HSCROLL handling. The scroll bar and the horizontal extent set by Show are not reset; send the LB_SETHORIZONTALEXTENT message with a width of 0 to the control, and hide the bar with ShowScrollBar, if clearing them is required.

Class_Terminate removes the subclass with the same internal helper, so the subclass is removed when the cListboxHScroll instance is destroyed.

Example

This example attaches a horizontal scrollbar to a ListBox and removes it when the form closes.

Private HScroll As cListboxHScroll

Private Sub Form_Load()
    Set HScroll = New cListboxHScroll

    List1.AddItem "Alpha"
    List1.AddItem "Beta -- a much longer entry that would normally be clipped"
    List1.AddItem "Gamma"

    HScroll.Show List1
End Sub

Private Sub Form_Unload(Cancel As Integer)
    HScroll.UnShow
End Sub

Remarks

cListboxHScroll uses window subclassing via the comctl32 SetWindowSubclass / RemoveWindowSubclass / DefSubclassProc API family. The subclass is identified by the constant ID 1 in every instance.

Note 1.0.0.1

Two instances on the same ListBox install two subclasses, and each handles every WM_HSCROLL message: a click on a scroll arrow moves the thumb 40 pixels, not 20. UnShow of one instance removes only its own subclass. Use one instance per ListBox.

Instances on different list boxes are independent: UnShow of one leaves the scroll handling of the other in place.

The class stores the ListBox reference in a class-level variable (Ctl). Replacing the control by calling Show with a different ListBox does not automatically remove the subclass from the first control; call UnShow before switching targets.