HOWTO: Create a System-Modal Program/Window in Visual Basic

ID: Q147817

The information in this article applies to:

SUMMARY

From a Microsoft Visual Basic for Windows program under Windows 95, Windows 98, or Windows 3.x, you can disable the ability to switch to other Windows programs by calling the Windows API function SetSysModalWindow. The method outlined below will not work with Windows NT.

NOTE: An equivalent Win32 API function for SetSysModalWindow does not exist. System modal windows contradict the concept of multitasking and thus are not implemented in Win32.


MORE INFORMATION

Microsoft Windows is designed so that the user can switch between applications without terminating one program to run another program. There may be times when the program needs to take control of the entire environment and run from only one window, restricting the user from switching to any other application. An example of this is a simple security system, or a time-critical application that may need to go uninterrupted for long periods of time.

Passing the handle to the window through the argument of SetSysModalWindow will limit the user to that particular window. This will not allow the user to move to any other applications with the mouse or use ALT+ESC or CTRL+ESC to bring up the Task Manager. You can even remove the system menu if you do not want the user to exit through the ALT+F4 (Close) combination.

All child windows that are created by the system-modal window become system- modal windows. When the original window becomes active again, it is system- modal. To end the system-modal state, destroy the original system-modal window.

Care must be taken when using the SetSysModalWindow API from within the Visual Basic for Windows programming environment. Pressing CTRL+BREAK to get to the [break] mode leaves your modal form with no way to exit unless you restart your system. When using the SetSysModalWindow within the environment, be sure to exit your application by destroying the window with either the ALT+F4 in the system menu, or by some other means from within your running program.

To use the SetSysModalWindow API function, declare the API call in your global section, as follows: 


   Declare Function SetSysModalWindow Lib "User" (ByVal hwnd%) As Integer

At an appropriate place in your code, add the following: 

   Success% = SetSysModalWindow(hwnd)

Once this line is executed, your window will be the only window that can get focus until that window is destroyed.

NOTE: Because Visual Basic for Windows was not designed with system modal capabilities in mind, using a MsgBox, InputBox, or Form.Show of another form from a system modal window will not work correctly. If you want to show another window from a system modal form, use another Visual Basic for Windows form and call SetSysModalWindow for this second form also, so that it becomes the system modal window. When the second form is unloaded, the original system modal form will again become the system modal window. Note that because the window(s) shown from a system modal window must also call SetSysModalWindow, and since MsgBox/InputBox windows cannot have associated code, you should not call the MsgBox or InputBox functions from a system modal window.

Additional query words: kbVBp400 kb16only kbVBp kbDSupport kbdsd kbNoKeyWord 


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Platform          : WINDOWS 
Issue type        : kbhowto

Last Reviewed: June 8, 1999