How To Hook Into a Window's Messages Using AddressOf

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 allows sub-classing through the use of the AddressOf operator. This article's purpose is to demonstrate a hooked message stream.

NOTE: It is beyond the scope of this article to describe any particular application of this technique.

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.

CAUTION: ANY USE OF THE SAMPLE CODE PROVIDED IN THIS ARTICLE IS AT YOUR OWN RISK. Microsoft provides programming examples for illustration only, without warranty either expressed or implied, including, but not limited to, the implied warranties of merchantability and/or fitness for a particular purpose. This article assumes that you are familiar with the programming language being demonstrated and the tools used to create and debug procedures. Microsoft support professionals can help explain the functionality of a particular procedure, but they will not modify these examples to provide added functionality or construct procedures to meet your specific needs.
If you have limited programming experience, you may want to contact a Microsoft Certified Partner or Microsoft Advisory Services. For more information, visit these Microsoft Web sites:

Microsoft Certified Partners - https://partner.microsoft.com/global/30000104 (https://partner.microsoft.com/global/30000104)

Microsoft Advisory Services - http://support.microsoft.com/gp/advisoryservice (http://support.microsoft.com/gp/advisoryservice)

For more information about the support options that are available and about how to contact Microsoft, visit the following Microsoft Web site:http://support.microsoft.com/default.aspx?scid=fh;EN-US;CNTACTMS (http://support.microsoft.com/default.aspx?scid=fh;en-us;cntactms) 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.

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.

The 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.

Microsoft Visual Basic Books Online (http://msdn.microsoft.com/vbasic/)

"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/instmsi.htm)

Win32 Programmer Reference: CallWindowProc; SetWindowLong