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

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.

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.

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 (http://www.microsoft.com/msdownload/platformsdk/sdkupdate/downlevel.htm)

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