Article: Q174301
Product(s): Microsoft Visual Basic for Windows
Version(s):
Operating System(s):
Keyword(s): kbGrpDSVB
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
- Microsoft Visual Basic Control Creation Edition for Windows, version 5.0
- Microsoft Visual Basic Learning Edition for Windows, version 5.0
- Microsoft Visual Basic Professional Edition for Windows, version 5.0
- Microsoft Visual Basic Enterprise Edition for Windows, version 5.0
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SUMMARY
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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
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For additional information, please see the following article in the Microsoft
Knowledge Base:
Q168795 : HOWTO: Hook Into a Window's Messages Using AddressOf
Additional query words: kbVBp500 kbVBp600 kbVBp kbdsd kbDSupport kbControl
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Keywords : kbGrpDSVB
Technology : kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbVB500Search kbVB600Search kbVBA500Search kbVBA500 kbVBA600 kbVB500 kbVB600 kbZNotKeyword3
Issue type : kbhowto
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