HOWTO: Intercept Keyboard Input from Visual Basic

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

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:

   ARTICLE-ID: Q74905
   TITLE     : PRB: Access Key Causes Different Event Order than Mouse
               Click

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 CommandButton and note the sequence of events that occur as show 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 on a system, but not using "pure" Visual Basic. The hook must be placed in a standard DLL; while Visual Basic 5.0 can create OLE DLLs, it cannot create standard DLLs.

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