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Tip of the Day
Language: Visual Basic
Expertise: Advanced
Apr 23, 1999

Create See-Through Controls

Most standard controls send WM_CTLCOLORXXXXX messages to their parent when they're about to draw themselves. VB normally handles these messages and responds appropriately with the ForeColor and BackColor properties that you have set for the control. However, it's possible to override the standard behavior and achieve a transparent background with several basic control types. This example uses the ubiquitous MsgHook for subclassing, but you can use whichever method you prefer. You must ensure that ClipControls is set to False for the form; otherwise, you'll see whatever is below your form as the background for the controls instead of the background bitmap:
 
Option Explicit

Private Declare Function SetBkMode Lib "gdi32" (ByVal hdc _
	As Long, ByVal nBkMode As Long) As Long
Private Declare Function SetTextColor Lib "gdi32" _
	(ByVal hdc As Long, ByVal crColor As Long) As Long
Private Declare Function GetStockObject Lib "gdi32" _
	(ByVal nIndex As Long) As Long

Private Const WM_CTLCOLORSTATIC = &H138
Private Const TRANSPARENT = 1
Private Const NULL_BRUSH = 5

Private Sub Form_Load()
	' Me.ClipControls = False
	' Must be set at design-time!
	Msghook1.HwndHook = Me.hWnd
	Msghook1.Message(WM_CTLCOLORSTATIC) = True
End Sub

Private Sub Msghook1_Message(ByVal msg As Long, ByVal wp _
	As Long, ByVal lp As Long, result As Long)
	Select Case msg
		Case WM_CTLCOLORSTATIC
			' Call the original windowproc to handle the
			' foreground colour for the Controls etc
			Call Msghook1.InvokeWindowProc(msg, wp, lp)

			' Set the background mode to transparent
			Call SetBkMode(wp, TRANSPARENT)

			' Get the stock null brush and return it
			' The brush does nothing when the control
			' paints using it, hence giving the
			' transparency effect
			result = GetStockObject(NULL_BRUSH)

		Case Else
			' [Replace this line with your own code 
			' to call the original windowproc]
			result = Msghook1.InvokeWindowProc(msg, wp, lp)
	End Select
End Sub
This code works for option buttons and checkboxes. I haven't found any side effects yet. You can make other controls transparent in a similar way, but some of them-including textboxes and listboxes-don't work correctly because the background doesn't get erased. You can probably get them to work correctly with additional code. Frames are even harder to get to work correctly; I resorted to using a button created using CreateWindowEx with the BS_GROUPBOX style. Even then, I had to return a brush of approximately the color of the background image; otherwise, you could see the line under the text for the frame.
Jeremy Adams
 
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