HOWTO: Hook Into a Window's Messages Using AddressOf

• Microsoft Visual Basic Learning, Professional, and Enterprise Editions for Windows, version 6.0
• Microsoft Visual Basic Enterprise Edition for Windows, version 5.0

SUMMARY

Hooking into a Window (sometimes called sub-classing) is a technique that enables interception of the messages that are being sent to that Window. Microsoft Visual Basic version allows sub-classing through the use of the AddressOf operator.

NOTE: This article's purpose is to demonstrate a hooked message stream. It is beyond the scope of this article to describe any particular application of this technique.

WARNING: ANY USE BY YOU OF THE SAMPLE CODE PROVIDED IN THIS ARTICLE IS AT YOUR OWN RISK. Microsoft provides this sample code "as is" without warranty of any kind, either express or implied, including but not limited to the implied warranties of merchantability and/or fitness for a particular purpose.

WARNING: Failure to unhook a window before its imminent destruction will result in application errors, Invalid Page Faults, and data loss. This is due the fact that the new WindowProc function being pointed to no longer exists, but the window has not been notified of the change. Always unhook the sub-classed window upon unloading the sub-classed form or exiting the application. This is especially important while debugging an application that uses this technique within the Microsoft Visual Basic Development Environment. Pressing the End button or selecting End from the Run menu without unhooking will cause an Invalid Page Fault and close Microsoft Visual Basic.

MORE INFORMATION

Microsoft Windows controls applications by sending messages to the windows that are created by each application. These messages alert the targeted window when it's time to redraw, when a mouse button is pressed, and all of the other information a window needs to know in order to act appropriately.

Thus, a minimal Windows application consists of a function for which these messages are processed (called WindowProc). This function is registered into the system when the window is created so the system knows where to send messages.

The following application consists of a simple form with two command buttons. The code is designed to intercept Windows messages being sent to the form and to print the values of those messages in the Immediate window.

Step-by-Step Example

1. Start a new Visual Basic Standard EXE project. Form1 is created by default.



2. Add two CommandButtons and a Standard Module to the form.



3. Add the following code to the Declarations section of Module1:




      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 IsHooked As Boolean
      Global lpPrevWndProc As Long
      Global gHW As Long

      Public Sub Hook()
          If IsHooked Then
          MsgBox "Don't hook it twice without " & _
            "unhooking, or you will be unable to unhook it."
          Else
          lpPrevWndProc = SetWindowLong(gHW, GWL_WNDPROC, _
          AddressOf WindowProc)
          IsHooked = True
          End If
      End Sub

      Public Sub Unhook()
          Dim temp As Long
          temp = SetWindowLong(gHW, GWL_WNDPROC,lpPrevWndProc)
          IsHooked = False
      End Sub

      Function WindowProc(ByVal hw As Long, ByVal uMsg As _
      Long, ByVal wParam As Long, ByVal lParam As Long) As Long
          Debug.Print "Message: "; hw, uMsg, wParam, lParam
          WindowProc = CallWindowProc(lpPrevWndProc, hw, _
          uMsg, wParam, lParam)
      End Function 

4. Add the following code to the Declarations section of Form1:




      Private Sub Form_Load()
          gHW = Me.hwnd
          Command1.Caption = "Hook"
          Command2.Caption = "Unhook"
      End Sub

      Private Sub Command1_Click()
          Hook
      End Sub

      Private Sub Command2_Click()
          Unhook
      End Sub

      Private Sub Form_QueryUnload(Cancel As Integer, UnloadMode  _
          As Integer)
          If IsHooked Then
          Cancel = 1
          MsgBox "Unhook before closing, or the IDE will crash."
          End If
      End Sub 

5. Before running this sample, save your project.



6. Press the F5 key to run the program, and click Hook. The Immediate window starts filling with the messages the form is receiving, hooked through to your new WindowProc function and then passed on to the Form's own handler.
   
   These two Hook and Unhook procedures enable the code to hook into the stream of messages.
   
   In the first procedure "Hook" you make use of the SetWindowLong function. The SetWindowLong function changes an attribute of a specified window. It takes the following parameters:
   
   
   • hwnd: The handle of the Window you are going to change.
   • nIndex: The action you are going to do to the window.
   • dwNewLong: The new value you change to.
   
   In this example, you use the Form's hwnd property as the targeted window to change. You then use the GWL_WNDPROC constant to tell the SetWindowLong function that you want to change the address of the target window's WindowProc function. Finally you set dwNewLong to the address of a new WindowProc function (see next step). Notice that you store the previous WindowProc address in the lpPrevWndProc variable.
   
   The second procedure "UnHook" simply reverses what you have done and puts the address of the original window procedure back.
   
   Here WindowProc is the function that you are routing the window messages to when you "Hook" the form's WindowProc function. Note that you make use of the CallWindowProc function; using the lpPrevWndProc variable to send any unprocessed messages to the original handler. Hence, you are allowing a chain of window procedures to process all messages.

REFERENCES

Microsoft Windows 32 SDK

Win32 Programmer Reference: CallWindowProc; SetWindowLong

Microsoft Visual Basic Books Online

Additional query words:

Keywords : kbAPI kbSDKPlatform kbVBp500 kbVBp600
Version : WINDOWS:5.0,6.0
Platform : WINDOWS
Issue type : kbhowto