Tip 66: Disabling Task-Switching with the SetSysModalWindow Function

Created: April 24, 1995

Abstract

When developing an application in Visual Basic®, you may need to perform a task that should not be interrupted. This article explains how to use the Windows® application programming interface (API) SetSysModalWindow and LockInput functions to disable task-switching.

Preventing a User from Switching to Another Application

The Windows® application programming interface (API) SetSysModalWindow function can be used to prevent a user from switching to a different Windows-based application while your program is executing. The ALT+TAB, CTRL+ESC, ALT+F4, and ALT+ESC keystroke combinations will not bring up the Task Manager or any other application—these keystrokes will simply be ignored. For a discussion of the SetSysModalWindow function, see "Additional References" below.

In addition, the LockInput function can be used to force all input to your Visual Basic® application only. No other application will receive any mouse or keyboard data. The Declare statement for the LockInput function is as follows (note that it must be typed as a single line of code):

Declare Function LockInput Lib "User" (ByVal hReserved As Integer, ByVal 
   hwndInput As Integer, ByVal fLock As Integer) As Integer

The LockInput function requires three arguments:

When your program is terminated, you must use the LockInput function to restore input to other Windows-based programs. In addition, you must destroy the system modal window; otherwise the user will not be able to switch to any other Windows-based applications and will have to reboot the computer system.

Example Program

The program below shows how you can prevent a user from switching to another Windows-based application while your program is executing.

1. Create a new project in Visual Basic. Form1 is created by default.
   
   
2. Set the following properties for Form1:

   ClipControls = False
   ControlBox = false
   MaxButton = False
   MinButton = False
   

3. Add the following Dim and Declare statements to the General Declarations section of Form1 (note that each Declare statement must be typed as a single line of code):

   Declare Function GetActiveWindow Lib "User" () As Integer
   Declare Function SetFocusAPI Lib "User" Alias "SetFocus" (ByVal Hwnd As Integer) 
      As Integer
   Declare Function SetSysModalWindow Lib "User" (ByVal Hwnd As Integer) As Integer
   Declare Function LockInput Lib "User" (ByVal hReserved As Integer, ByVal 
      hwndInput As Integer, ByVal fLock As Integer) As Integer
   Dim TopHwnd As Integer
   

4. Add the following code to the Form_Load event for Form1:

   Sub Form_Load()
   
     Dim X As Integer
       
     Show
     DoEvents
     TopHwnd = GetActiveWindow()
       
     X = SetFocusAPI(TopHwnd)
     X = SetSysModalWindow(TopHwnd)
     X = LockInput(0, TopHwnd, 1)
   End Sub
   

5. Add the following code to the Form_Unload event for Form1:

   Sub Form_Unload(Cancel As Integer)
     X = LockInput(0, TopHwnd, 0)
   End Sub
   

6. Add a Command Button control to Form1. Command1 is created by default. Set its Caption property to "Quit".
   
   
7. Add the following code to the Click event for Command1:

   Sub Command1_Click()
       X = LockInput(0, TopHwnd, 0)
       End
   End Sub
   

Additional References

Knowledge Base Q72674. "How to Create a System-Modal Program/Window in Visual Basic." (Development Library, Knowledge Base and Bug Lists)