How To Position a MsgBox Using a Windows Hook Procedure

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

  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:

168795 : How To Hook Into a Window's Messages Using AddressOf


170570 : How To Build a Windows Message Handler with AddressOf in VB5
Properties

Article ID: 180936 - Last Review: Mar 11, 2005 - Revision: 1

Feedback