What can cause Windows to unhook a low level (global) keyboard hook?

Question

We have some global keyboard hooks installed via SetWindowsHookEx with WH_KEYBOARD_LL that appear to randomly get unhooked by Windows.

We

Answer 1

I think this has to be a timeout issue.

Other developers have reported a Windows7 specific problem with low level hooks being unhooked if they exceed an (undocumented) timeout value.

See this thread for other developers discussing the same problem. It may be that you need to perform a busy loop (or a slow Garbage Collection) rather than a Sleep to cause the unhooking behavior. A breakpoint in the LowLevelKeyboardProc function might also create timeout problems. (There's also the observation that a heavy CPU load by another task might provoke the behavior - presumably because the other task steals CPU cycles from the LowLevelKeyboardProc function and causes it to take too long.)

The solution suggested in that thread is to try setting the LowLevelHooksTimeout DWORD value in the registry at HKEY_CURRENT_USER\Control Panel\Desktop to a larger value.

Remember that one of the glories of C# is that even simple statements can take an inordinate amount of time if a garbage collection occurs.. This (or CPU loading by other threads) might explain the intermittent nature of the problem.