HOWTO: Display Text on a Dithered Background in a TextBox

• Microsoft Visual Basic Control Creation, Learning, Professional, and Enterprise Editions for Windows, version 5.0

SUMMARY

When a dithered color is selected as the background of a TextBox, the background around the text is displayed as a different, solid color when the display is set for 256 colors or less. To work around this limitation, you must hook the WM_CTLCOLOREDIT message and set the text background color to TRANSPARENT. This is not a trivial alternative, and using a solid background color is recommended if at all possible.

MORE INFORMATION

WARNING: ANY USE BY YOU OF THE SAMPLE CODE PROVIDED IN THIS ARTICLE IS AT YOUR OWN RISK. Microsoft provides this sample code "as is" without warranty of any kind, either express or implied, including but not limited to the implied warranties of merchantability and/or fitness for a particular purpose.

WARNING: Failure to unhook a window before its imminent destruction will result 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 5.0 Development Environment. Pressing the END button or selecting End from the Run menu without unhooking will cause an Invalid Page Fault and close Microsoft Visual Basic.

A TextBox control cannot draw text with a dithered background color, but it can draw text with a transparent background color. If the background of the control is dithered, this achieves the same visual effect. (Note the distinction between the background of the text and the background of the control. It is simple to get the control's background dithered; the problem is getting the text background dithered as well.)

From the SDK perspective, the answer is simple; handle the WM_CTLCOLOREDIT message in the parent and call SetBkMode() to set the background mode to TRANSPARENT. This is less simple in Visual Basic.

The following application consists of a form with a single Textbox control and a code module:

1. Create a new Visual Basic "Standard EXE" project.

2. Add a Textbox to Form1.

3. Paste the following code into Form1's code module:

         Private Sub Form_Load()
            gHW = Me.hwnd
            Text1.BackColor = &HC0E0FF
            Hook
         End Sub
   
         Private Sub Form_Unload(Cancel As Integer)
            Unhook
         End Sub
   
   

4. Add a module.

5. Paste the following code into this module:

         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
   
         Declare Function SetBkMode Lib "gdi32" _
            (ByVal hdc As Long, ByVal nBkMode As Long) As Long
   
         Public Const WM_CTLCOLOREDIT = &H133
         Public Const TRANSPARENT = 1
         Public Const GWL_WNDPROC = -4
   
         Global lpPrevWndProc As Long
         Global gHW As Long
   
         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_CTLCOLOREDIT Then
               i = SetBkMode(wParam, TRANSPARENT)
            End If
            WindowProc = CallWindowProc(lpPrevWndProc, hw, uMsg, wParam, _
               lParam)
         End Function
   
   

6. Save the project, and then run it.

   RESULT: When the form is displayed, you will see a TextBox with a dithered background. Type into the TextBox and the dithered background is maintained.

7. To terminate the application, use Alt-F4, the control box menu, or click the close button on the form. If you click the stop button in the Visual Basic Design environment, the Unload event is not triggered to unhook the window and an IPF will occur.

REFERENCES

For additional information, please see the following article in the Microsoft Knowledge Base:

   ARTICLE-ID: Q16879
   TITLE     : HOWTO: Hook Into a Window's Messages Using AddressOf

Keywords          : VBKBCtrl VBKBInt VBKBStd VBKBTextBox VBKBWinAPI
Version           : WINDOWS:5.0
Platform          : WINDOWS
Issue type        : kbhowto