How To Build a Windows Message Handler with AddressOf in Visual Basic

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Article ID: 170570 - View products that this article applies to.
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SUMMARY

This article demonstrates how to build a Windows message handler in Visual Basic. It traps the right-click message and replaces the default context menu with a custom built one.

MORE INFORMATION

Prior to the release of Visual Basic 5.0, many developers used third-party Windows message-handling tools or developed their own with tools such as Microsoft Visual C++. With the addition of the AddressOf function to Visual Basic 5.0 and higher, developers can now create their own Windows message handling routines within their Visual Basic applications.

For example, when a user right-clicks on a textbox in Windows 95, Windows 98, Windows Me, Windows NT 4.0 or Windows 2000, the operating system automatically displays a default context menu. This default behavior occurs before the Visual Basic application fires the MouseUp event. Without the use of a Windows message handler, there is no way to replace the default context menu with a custom built context menu.

The following code shows how to build a Windows message handler to trap and discard the right-click message and to replace the default context menu with a custom built one.

WARNING: Using AddressOf may cause unpredictable results if you don't completely understand the concept of function callbacks. You must understand how the basic portion of the callback works, and also the code of the DLL into which you are passing your function address. Debugging such interactions is difficult because the program runs in the same process as the development environment. In some cases, systematic debugging may not be possible. See details in the REFERENCES section of this article for more information.

Step-by-Step Example

  1. Create a new Standard EXE project. Form1 is created by default.
  2. Use the Menu Editor to add two menu items to the form:
    1. For the first menu item, set its Caption property to "My Popup," its Name property to "mnuPopup," and its Visible property to "False."
    2. For the second menu item, set its Caption property to "My Context," its Name property to "mnuPopupContext," and use the arrow keys to indent the item to be a submenu of the first item.
  3. Add two CommandButtons and a textbox to the form:
    1. For the first CommandButton, set its Name property to "cmdHook" and its Caption property to "&Hook."
    2. For the second CommandButton, set its Name property to "cmdUnHook" and its Caption property to "&UnHook."
    3. For the textbox, set its Name property to "txtHook" and its Text property to "Right Click On Me!"
  4. Add the following code to the form:
          Private Sub cmdHook_Click()
              Hook
          End Sub
    
          Private Sub cmdUnHook_Click()
              UnHook
          End Sub
    
          Private Sub Form_Load()
              gHW = txtHook.hWnd
          End Sub
    
          Private Sub txtHook_MouseUp(Button As Integer, Shift As Integer, _
                                      X As Single, Y As Single)
              If Button = vbRightButton Then
                  PopupMenu mnuPopup
              End If
          End Sub
    
    					
  5. Add a Module to the project.
  6. Add the following code to the new 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
    
        Public Const GWL_WNDPROC = -4
    
        Public Const WM_RBUTTONUP = &H205
    
        Global lpPrevWndProc As Long
        Global gHW As Long
    
        Public Sub Hook()
            lpPrevWndProc = SetWindowLong(gHW, GWL_WNDPROC, _
                                         AddressOf WindowProc)
        End Sub
    
        Public Sub UnHook()
            Dim lngReturnValue As Long
            lngReturnValue = 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
    
            Select Case uMsg
                Case WM_RBUTTONUP
                    Form1.PopupMenu Form1.mnuPopup
                Case Else
                    WindowProc = CallWindowProc(lpPrevWndProc, hw, _
                                               uMsg, wParam, lParam)
            End Select
        End Function
    
    					
  7. Save the project and run it. Right-click on the textbox and notice that the default context menu appears before the custom menu. Click on the CommandButton marked "Hook" to enable the Windows message handler. Right-click on the textbox and note that the default context menu no longer appears. Be sure to click on the CommandButton marked "UnHook" before quitting the application. Always disable a custom Windows message handler before the application terminates. Clicking the End button, while the Windows message handler is still enabled, will cause an IPF.

REFERENCES

If you are not familiar with the use of callbacks, then the following references may help:

"Dan Appleman's Visual Basic Programmer's Guide to the Win32 API" by Dan Appleman

Platform Software Development Kit (SDK) Online Help

(c) Microsoft Corporation 1997, All Rights Reserved. Contributions by David Sceppa, Microsoft Corporation

Properties

Article ID: 170570 - Last Review: June 29, 2004 - Revision: 2.3
APPLIES TO
  • Microsoft Visual Basic 5.0 Learning Edition
  • Microsoft Visual Basic 6.0 Learning Edition
  • Microsoft Visual Basic 5.0 Professional Edition
  • Microsoft Visual Basic 6.0 Professional Edition
  • Microsoft Visual Basic 5.0 Enterprise Edition
  • Microsoft Visual Basic Enterprise Edition for Windows 6.0
Keywords: 
kbapi kbhowto KB170570

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