Subclassing and Hooking with Visual Basic

Caveats

Once again, performance might be an issue with the use of the WH_MOUSE or WH_MOUSE_LL hook, especially with system-wide and low-level mouse hooks. You should use these hooks judiciously. For more information on the problems with low-level hooks, refer to Section 12.6 of Chapter 12.

When using the low-level mouse hook (WH_MOUSE_LL), note that the debug hook cannot receive the input to this hook before the hook is called. The debug hook works normally with the mouse hook (WH_MOUSE). The debug hook is described in Chapter 21.

A registry key is associated with low-level hooks, such as WH_MOUSE_LL and WH_KEYBOARD_LL (see Chapter 12). The numeric registry value LowLevelHooksTimeout is located in the following registry key:

HKEY_CURRENT_USER\Control Panel\Desktop

This value controls the timeout period in milliseconds for all low-level hooks. If the filter function for a low-level hook does not return within this time span, the system will call the next low-level hook in the chain.

Although all 32-bit and 64-bit operating systems support the WH_MOUSE hook, all do not support the WH_MOUSE_LL hook. The WH_MOUSE_LL hook is found only in Windows NT (Service Pack 3 or higher) and Windows 2000; it is not found in Windows 9x.

As one final caution, there is a bug when using the mouse hooks from within a VB user control. Article Q238672 in the Microsoft Developer Network (MSDN) details the bug and how to work around it (Visual Studio 6, Service Pack 4 solves this problem).