KnowledgeBase Archive

An Archive of Early Microsoft KnowledgeBase Articles

View on GitHub

Q177992: HOWTO: Intercept Keyboard Input from Visual Basic

Article: Q177992
Product(s): Microsoft Visual Basic for Windows
Version(s): WINDOWS:5.0,6.0
Operating System(s): 
Keyword(s): kbVBp500 kbVBp600 kbGrpDSVB
Last Modified: 11-JAN-2001

-------------------------------------------------------------------------------
The information in this article applies to:

- Microsoft Visual Basic Learning Edition for Windows, versions 5.0, 6.0 
- Microsoft Visual Basic Professional Edition for Windows, versions 5.0, 6.0 
- Microsoft Visual Basic Enterprise Edition for Windows, versions 5.0, 6.0 
-------------------------------------------------------------------------------

SUMMARY
=======

This article demonstrates how to intercept input from the keyboard before it
reaches your Visual Basic application. Most keyboard input can be readily
intercepted by a Visual Basic application by using the KeyDown event of the form
or control. The technique shown here allows you how to handle otherwise
unavailable key combinations such as the TAB key or access key combinations.

MORE INFORMATION
================

You can create a keyboard hook that intercepts all keyboard input directed to a
given thread. To create this hook, you must replace the default KeyboardProc()
function with a KeyboardProc() of your own. The AddressOf() construct introduced
in Visual Basic 5.0 makes this possible. Within your KeyboardProc(), you must
either pass along the keyboard input you receive or delete it. You cannot change
the contents of the message you receive.

Why would you want to do this? One purpose of the keyboard hook could be to
enforce data validation before the current control loses focus. Another purpose
is to work around the difference in event sequence when using an access (or hot
key) as opposed to a TAB character or mouse click. For additional information on
this problem, please see the following article in the Microsoft Knowledge Base:

  Q74905 PRB: Access Key Causes Different Event Order than Mouse Click

The next section illustrates how to create a sample application that works around
this behavior by intercepting the access key, setting the focus, and then
causing a mouse-click event to occur. Please note that this does not resolve all
issues with the key order problems. For example, calling Message boxes from an
event affects the event order as well.

WARNING: Failure to unhook the keyboard hook before exiting your application will
result in errors, Invalid Page Faults, and data loss. This is caused by the
system pointing to the your KeyboardProc() function, which no longer exists
because it has not been notified of the change. Always unhook the keyboard upon
exiting the application. This is especially important while debugging a program
that uses this technique within the Microsoft Visual Basic 5.0 Development
Environment. Clicking the End button or selecting End from the Run menu without
unhooking will cause an Invalid Page Fault and close Microsoft Visual Basic
without giving you a chance to save your code.

Steps to Create Sample Project
------------------------------

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

2. Add a TextBox and a CommandButton to Form1. Set the TabIndex of the TextBox
  to 0.

3. Add a module to the project. From the Project menu, click Add Module.

4. Copy the following code to the Code window of Module1:

  Public Declare Function CallNextHookEx Lib "user32" _
     (ByVal hHook As Long, _
     ByVal nCode As Long, _
     ByVal wParam As Long, _
     ByVal lParam As Long) As Long

  Public Declare Function UnhookWindowsHookEx Lib "user32" _
     (ByVal hHook As Long) 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 PostMessage Lib "user32" _
     Alias "PostMessageA" _
     (ByVal hwnd As Long, _
     ByVal wMsg As Long, _
     ByVal wParam As Long, _
     ByVal lParam As Long) As Long

  Public Const WH_KEYBOARD = 2
  Public Const KBH_MASK = &H20000000
  Public Const WM_LBUTTONDOWN = &H201
  Public Const WM_LBUTTONUP = &H202

  Global hHook As Long

  Public Function KeyboardProc(ByVal nCode As Long, ByVal wParam As Long, _
                                ByVal lParam As Long) As Long
     If nCode >= 0 Then
     'Process keys you want to filter
        If wParam = Asc("C") And (lParam And KBH_MASK) <> 0 Then
           If (lParam And &HC0000000) = 0 Then
              Form1.Command1.SetFocus
                 Call PostMessage(Form1.Command1.hwnd, _
                                   WM_LBUTTONDOWN, 0, &H20002)
                 Call PostMessage(Form1.Command1.hwnd, _
                                   WM_LBUTTONUP, 0, &H20002)
                 KeyboardProc = 1
                 Exit Function
           End If
        End If
     End If
     KeyboardProc = CallNextHookEx(hHook, nCode, wParam, lParam)
  End Function

5. Copy the following code to the Code window of the Form1 form:

  Option Explicit

  Private Sub Command1_Click()
     Debug.Print "Command1_Click"
  End Sub

  Private Sub Command1_GotFocus()
     Debug.Print "Command1_GotFocus"
  End Sub

  Private Sub Form_Load()
     hHook = SetWindowsHookEx(WH_KEYBOARD, _
                              AddressOf KeyboardProc, 0&, App.ThreadID)
  End Sub

  Private Sub Form_Unload(Cancel As Integer)
     Call UnhookWindowsHookEx(hHook)
  End Sub

  Private Sub Text1_LostFocus()
     Debug.Print "Text1_LostFocus"
  End Sub

6. Press the F5 key to run the program.

The text box should have the focus. Click the command button and note the
sequence of events that occur as shown in the debug window:

  Text1_LostFocus
  Command1_GotFocus
  Command1_Click

Set the focus to the text box and press the ALT+C keys and note that the same
events occur.

Notes
-----

Hooks do not always behave the same way in the IDE as they do in an EXE. Make
certain that you test your solution in an EXE before you move on to other parts
of your project.

It is possible to intercept all of the keyboard input from all of the
applications running on a system, but not using "pure" Visual Basic. The hook
must be placed in a standard DLL; while Visual Basic can create OLE DLLs, it
cannot create standard DLLs.

(c) Microsoft Corporation 1997, All Rights Reserved.
Contributions by Arsenio Locsin, Microsoft Corporation


Additional query words: subclass

======================================================================
Keywords          : kbVBp500 kbVBp600 kbGrpDSVB 
Technology        : kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbVB500Search kbVB600Search kbVBA500 kbVBA600 kbVB500 kbVB600
Version           : WINDOWS:5.0,6.0
Issue type        : kbhowto

=============================================================================

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.