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How to disable task switching on Win32 platforms

Article ID: 226359 - View products that this article applies to.

This article was previously published under Q226359

SUMMARY

This article describes how to disable task switching and other system functions accessed through key combinations such as CTRL+ESC and ALT+TAB on Win32 Platforms.

Windows 95 and Windows 98

Applications can enable and disable ALT+TAB and CTRL+ESC, for example, by calling SystemParametersInfo (SPI_SETSCREENSAVERRUNNING). To disable ALT+TAB and CTRL+ESC, set the uiParam parameter to TRUE; to enable the key combinations, set the parameter to FALSE:

UINT nPreviousState;

// Disables task switching
SystemParametersInfo (SPI_SETSCREENSAVERRUNNING, TRUE, &nPreviousState, 0);

// Enables task switching
SystemParametersInfo (SPI_SETSCREENSAVERRUNNING, FALSE, &nPreviousState, 0);

Note Applications that use SystemParametersInfo (SPI_SETSCREENSAVERRUNNING) to disable task switching must enable task switching before exiting or task switching remains disabled after the process terminates.

Windows NT 4.0 Service Pack 3 and Later and Windows 2000

Applications can disable ALT+TAB or CTRL+ESC by installing a low-level keyboard hook. A low-level keyboard hook (WH_KEYBOARD_LL) is installed by calling SetWindowsHookEx. For more information on Window hooks see the "Hooks" overview in the Platform SDK documentation.

The following is a sample low-level keyboard hook procedure that disables CTRL+ESC, ALT+TAB, and ALT+ESC:

LRESULT CALLBACK LowLevelKeyboardProc (INT nCode, WPARAM wParam, LPARAM lParam)
{
    // By returning a non-zero value from the hook procedure, the
    // message does not get passed to the target window
    KBDLLHOOKSTRUCT *pkbhs = (KBDLLHOOKSTRUCT *) lParam;
    BOOL bControlKeyDown = 0;

    switch (nCode)
    {
        case HC_ACTION:
        {
            // Check to see if the CTRL key is pressed
            bControlKeyDown = GetAsyncKeyState (VK_CONTROL) >> ((sizeof(SHORT) * 8) - 1);
            
            // Disable CTRL+ESC
            if (pkbhs->vkCode == VK_ESCAPE && bControlKeyDown)
                return 1;

            // Disable ALT+TAB
            if (pkbhs->vkCode == VK_TAB && pkbhs->flags & LLKHF_ALTDOWN)
                return 1;

            // Disable ALT+ESC
            if (pkbhs->vkCode == VK_ESCAPE && pkbhs->flags & LLKHF_ALTDOWN)
                return 1;

            break;
        }

        default:
            break;
    }
    return CallNextHookEx (hHook, nCode, wParam, lParam);
}

Windows NT 4.0 Service Pack 2 and Earlier, Windows NT 3.51 and Earlier

Applications can disable CTRL+ESC system-wide by replacing the Windows NT Task Manager, but this is not recommended.

Applications can disable ALT+TAB and ALT+ESC when the application is running by registering hotkeys for the ALT+TAB and ALT+ESC combinations by calling RegisterHotKey.

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MORE INFORMATION

Older development tools, such as Microsoft Visual C++ Version 5.0 and earlier, might not contain the header files necessary to build an application that uses low-level keyboard hooks. To obtain the most recent header files, download the latest Platform SDK from the following Microsoft Web site:

http://www.microsoft.com/msdownload/platformsdk/sdkupdate/psdk-full.htm

(http://www.microsoft.com/msdownload/platformsdk/sdkupdate/psdk-full.htm)

Because low-level keyboard hooks are a feature specific to Windows NT 4.0 Service Pack 3 and later, define _WIN32_WINNT >= 0x0400 prior to including winuser.h (or windows.h).

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