software / rapidq / Examples / Devices / keyboard / Disable low level keys / DisableLowLevelKeys.txt

Search 搜索
Home Home

There are many situations when it's need to disable some combinations of keys from a VB program. For instance,  ALT-TAB, CTRL-ESC, ALT-ESC or others like these. Other combinations could be tested at form level using KeyPreview property and KeyPress / KeyDown / KeyUp events. All system keystrokes won't fire key events in a form (or other controls) because they are handled internally by the system. Since application threads never receive messages for these keystrokes, there is no way that an application can intercept them and prevent the normal processing. This behavior is "by design" and ensures that a user can always switch to another application’s window even if an application’s thread enters an infinite loop or hangs.

The question is how we can intercept this keystrokes? The solution could be achieved using hooks. A hook is a point in the Microsoft Windows message-handling mechanism where an application can install a subroutine to monitor the message traffic in the system and process certain types of messages before they reach the target window procedure.
For Windows NT SP3 (or higher), Microsoft introduced a new hook: WH_KEYBOARD_LL. This hook is called the low-level hook because it is notified of keystrokes just after the user enters them and before the system gets a chance to process them. This hook has a serious drawback: the thread processing the hook filter function could enter an infinite loop or hang. If this happens, then the system will no longer process keystrokes properly and the user will become incredibly frustrated. To alleviate this situation, Microsoft places a time limit on low-level hooks. When the system sends a notification to a low-level keyboard hook’s filter function, the system allows that function a fixed amount of time to execute. If the function does not return in the allotted time, the system ignores the hook filter function and processes the keystroke normally. The amount of time allowed (in milliseconds) is set via the LowLevelHooksTimeout value under the following registry subkey: HKEY_CURRENT_USER\Control Panel\Desktop.

The program (VB) is disabling some of these combinations (ALT-TAB, CTRL-ESC and ALT-ESC) as long as the option is checked.

Check it!

Ovidiu Crisan
© Sat 2024-6-22  Guidance Laboratory Inc. Hits:0 Last modified:2000-10-18 14:11:48