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Q244236: HOWTO: Use the System Color on ScrollBars Under Windows NT

Article: Q244236
Product(s): Microsoft Visual Basic for Windows
Version(s): WINDOWS:6.0
Operating System(s): 
Keyword(s): kbAPI kbColor kbCtrl kbScrollBar kbSDKWin32 kbVBp600 kbWndwMsg kbGrpDSVB kbDSupport
Last Modified: 11-JAN-2001

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The information in this article applies to:

- Microsoft Visual Basic Learning Edition for Windows, version 6.0 
- Microsoft Visual Basic Professional Edition for Windows, version 6.0 
- Microsoft Visual Basic Enterprise Edition for Windows, version 6.0 
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SUMMARY
=======

The background color of the horizontal and vertical scrollbar controls is white
under Microsoft Windows NT, while the system color is used under Microsoft
Windows 9.x. To use the system color on Windows NT, it is necessary to subclass
the form to prevent the WM_CTLCOLORSCROLBAR windows message from being processed
by the default windows procedure (WindowProc) of the form.

WARNING: Failure to unhook a window before its imminent destruction results in
application errors, Invalid Page Faults, and data loss. This is due to the fact
that the new WindowProc function being pointed to no longer exists, but the
window has not been notified of the change. Always unhook the sub-classed window
upon unloading the sub-classed form or exiting the application. This is
especially important while debugging an application that uses this technique
within the Microsoft Visual Basic Development Environment (IDE). Pressing the
END button or selecting END from the Run menu without unhooking can cause an
Invalid Page Fault and close Microsoft Visual Basic.

MORE INFORMATION
================

Microsoft Windows controls applications by sending messages to the windows that
are created by each application. These messages alert the targeted window when
it is time to redraw, when a mouse button is pressed, and all of the other
information a window needs to know in order to act appropriately.

Thus, a minimal Windows application consists of a function that processes these
messages (called WindowProc). This function is registered with the system when
the window is created so the system knows where to send messages.

The WM_CTLCOLORSCROLLBAR message is sent to the parent window of a scrollbar
control when the control is about to be drawn. By responding to this message,
the parent window can use the device context handle (hDC) to set the background
color of the scroll bar control. If this message is intercepted and not passed
on to the WindowProc, the scrollbar is set to the system color.

The following sample consists of a simple form with two CommandButtons and a
vertical scrollbar. It intercepts the WM_CTLCOLORSCROLLBAR message and discards
it.

NOTES:

- The scrollbar control flickers unless at least one other control is sited on
  the form. Setting the TabStop property of the VScrollBar to False prevents it
  from getting the focus.

- It is generally a good idea to start a hook from the Form_Load procedure and
  end it from the Form_Unload procedure. In this sample, CommandButtons are
  used so that the difference in the VScrollBar is more readily apparent.

Step-by-Step Example
--------------------

1. Start a new Visual Basic Standard EXE project. Form1 is created by default.

2. Add two CommandButtons and a VScrollBar to Form1. Set the TabStop property of
  VScrollBar1 to False.

3. On the Project menu, click Add Module to add a BAS Module to the project.

4. Add the following code to the General Declarations section of Module1:

  Option Explicit

  Declare Function CallWindowProc Lib "user32" Alias _
   "CallWindowProcA" (ByVal lpPrevWndFunc As Long, _
   ByVal hwnd As Long, ByVal Msg As Long, _
   ByVal wParam As Long, ByVal lParam As Long) As Long

  Declare Function SetWindowLong Lib "user32" Alias _
   "SetWindowLongA" (ByVal hwnd As Long, _
   ByVal nIndex As Long, ByVal dwNewLong As Long) As Long

  Public Const GWL_WNDPROC = -4
  public Const WM_CTLCOLORSCROLLBAR = 311

  Public Sub Hook()
     lpPrevWndProc = SetWindowLong(gHW, GWL_WNDPROC, _
     AddressOf WindowProc)
  End Sub

  Public Sub Unhook()
     Dim temp As Long
     temp = SetWindowLong(gHW, GWL_WNDPROC, lpPrevWndProc)
  End Sub

  Function WindowProc(ByVal hw As Long, ByVal uMsg As Long, _
                      ByVal wParam As Long, ByVal lParam As Long) As Long
     If uMsg <> WM_CTLCOLORSCROLLBAR Then
        Debug.Print uMsg, hw
        WindowProc = CallWindowProc(lpPrevWndProc, hw, uMsg, wParam, lParam)
     End If
  End Function

5. Add the following code to the General Declarations section of Form1:

  Option Explicit

  Dim lpPrevWndProc As Long
  Dim gHW As Long

  Private Sub Form_Load()
     gHW = Me.hwnd
     Command1.Caption = "Hook"
     Command2.Caption = "Unhook"
  End Sub

  Private Sub Command1_Click()
     Hook
     VScroll1.Refresh
  End Sub

  Private Sub Command2_Click()
     Unhook
  End Sub

6. Run the project and click the Hook button to hook the form. Observe that the
  scrollbar changes from white to the system color.

7. Click the Unhook button and then terminate the program.

REFERENCES
==========

  Q168795 HOWTO: Hook Into a Window's Messages Using AddressOf

Additional query words:

======================================================================
Keywords          : kbAPI kbColor kbCtrl kbScrollBar kbSDKWin32 kbVBp600 kbWndwMsg kbGrpDSVB kbDSupport 
Technology        : kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbVB600Search kbVBA600 kbVB600
Version           : WINDOWS:6.0
Issue type        : kbhowto

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