Q180936: HOWTO: Position a MsgBox Using a Windows Hook Procedure
Article: Q180936
Product(s): Microsoft Visual Basic for Windows
Version(s): WINDOWS:5.0,6.0
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, versions 5.0, 6.0
- Microsoft Visual Basic Professional Edition for Windows, versions 5.0, 6.0
- Microsoft Visual Basic Enterprise Edition for Windows, versions 5.0, 6.0
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SUMMARY
=======
This article illustrates how to position a message box on the screen.
MORE INFORMATION
================
You can create a CBT hook for your application so that it receives notifications
when windows are created and destroyed. If you display a message box with this
CBT hook in place, your application will receive a HCBT_ACTIVATE message when
the message box is activated. Once you receive this HCBT_ACTIVATE message, you
can position the window with the SetWindowPos API function and then release the
CBT hook if it is no longer needed.
Step-by-Step Example
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1. Start a new Standard EXE project. Form1 is created by default.
2. Add a module to the project and add the following code to the new module:
Type RECT
Left As Long
Top As Long
Right As Long
Bottom As Long
End Type
Public Declare Function UnhookWindowsHookEx Lib "user32" ( _
ByVal hHook As Long) As Long
Public Declare Function GetWindowLong Lib "user32" Alias _
"GetWindowLongA" (ByVal hwnd As Long, ByVal nIndex As Long) _
As Long
Public Declare Function GetCurrentThreadId Lib "kernel32" () As Long
Public Declare Function SetWindowsHookEx Lib "user32" Alias _
"SetWindowsHookExA" (ByVal idHook As Long, ByVal lpfn As Long, _
ByVal hmod As Long, ByVal dwThreadId As Long) As Long
Public Declare Function SetWindowPos Lib "user32" ( _
ByVal hwnd As Long, ByVal hWndInsertAfter As Long, _
ByVal x As Long, ByVal y As Long, ByVal cx As Long, _
ByVal cy As Long, ByVal wFlags As Long) As Long
Public Declare Function GetWindowRect Lib "user32" (ByVal hwnd _
As Long, lpRect As RECT) As Long
Public Const GWL_HINSTANCE = (-6)
Public Const SWP_NOSIZE = &H1
Public Const SWP_NOZORDER = &H4
Public Const SWP_NOACTIVATE = &H10
Public Const HCBT_ACTIVATE = 5
Public Const WH_CBT = 5
Public hHook As Long
Function WinProc1(ByVal lMsg As Long, ByVal wParam As Long, _
ByVal lParam As Long) As Long
If lMsg = HCBT_ACTIVATE Then
'Show the MsgBox at a fixed location (0,0)
SetWindowPos wParam, 0, 0, 0, 0, 0, _
SWP_NOSIZE Or SWP_NOZORDER Or SWP_NOACTIVATE
'Release the CBT hook
UnhookWindowsHookEx hHook
End If
WinProc1 = False
End Function
Function WinProc2(ByVal lMsg As Long, ByVal wParam As Long, _
ByVal lParam As Long) As Long
Dim rectForm As RECT, rectMsg As RECT
Dim x As Long, y As Long
'On HCBT_ACTIVATE, show the MsgBox centered over Form1
If lMsg = HCBT_ACTIVATE Then
'Get the coordinates of the form and the message box so that
'you can determine where the center of the form is located
GetWindowRect Form1.hwnd, rectForm
GetWindowRect wParam, rectMsg
x = (rectForm.Left + (rectForm.Right - rectForm.Left) / 2) - _
((rectMsg.Right - rectMsg.Left) / 2)
y = (rectForm.Top + (rectForm.Bottom - rectForm.Top) / 2) - _
((rectMsg.Bottom - rectMsg.Top) / 2)
'Position the msgbox
SetWindowPos wParam, 0, x, y, 0, 0, _
SWP_NOSIZE Or SWP_NOZORDER Or SWP_NOACTIVATE
'Release the CBT hook
UnhookWindowsHookEx hHook
End If
WinProc2 = False
End Function
3. Add two CommandButtons to Form1.
4. Add the following code to Form1:
Private Sub Command1_Click()
Dim hInst As Long
Dim Thread As Long
'Set up the CBT hook
hInst = GetWindowLong(Me.hwnd, GWL_HINSTANCE)
Thread = GetCurrentThreadId()
hHook = SetWindowsHookEx(WH_CBT, AddressOf WinProc1, hInst, _
Thread)
'Display the message box
MsgBox "This message box has been positioned at (0,0)."
End Sub
Private Sub Command2_Click()
Dim hInst As Long
Dim Thread As Long
'Set up the CBT hook
hInst = GetWindowLong(Me.hwnd, GWL_HINSTANCE)
Thread = GetCurrentThreadId()
hHook = SetWindowsHookEx(WH_CBT, AddressOf WinProc2, hInst, _
Thread)
'Display the message box
MsgBox "This message box is centered over Form1."
End Sub
5. Press the F5 key to run the program. Click Command1 and the message box
appears at the upper-left corner of the screen (0,0). Click OK to dismiss the
message box. Click Command2 and the message box appears at the center of
Form1. Click OK to dismiss the message box.
REFERENCES
==========
For additional information about creating message hooks with Visual Basic,
please see the following articles in the Microsoft Knowledge Base:
Q168795 : HOWTO: Hook Into a Window's Messages Using AddressOf
Q170570 : HOWTO: Build a Windows Message Handler with AddressOf in VB5
Additional query words: MessageBox kbVBp500 kbVBp600 kbVBp kbdsd kbDSupport KBWIN32SDK KBAPI
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Keywords : kbGrpDSVB
Technology : kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbVB500Search kbVB600Search kbVBA500 kbVBA600 kbVB500 kbVB600
Version : WINDOWS:5.0,6.0
Issue type : kbhowto
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