Subclassing and Hooking with Visual Basic

Description of Hooks

You can use the WH_MOUSE hook as a thread-specific hook, or you can place it in a dynamic link library (DLL) and use it as a system-wide hook. A thread-specific hook intercepts all messages within the thread in which it is installed. When you install a hook as a system-wide hook, it has to be placed in a DLL. This DLL is injected into every process so that the hook can operate on all messages in the system. Chapter 3 provides a thorough discussion of this concept.

The WH_MOUSE hook allows us to only watch or discard mouse messages. You can use this hook to intercept messages for a single thread or for all running threads. This hook provides a large amount of control over the mouse and the way the user is able to use the mouse.

The second mouse hook, WH_MOUSE_LL, was introduced with Windows NT Service Pack 3 and higher. This hook is similar to the WH_MOUSE hook, except that it intercepts messages at a much earlier point in the Windows messaging system. This provides extra information about mouse messages that is not available through the WH_MOUSE hook. Table 13-1 describes the information that you can obtain from each type of mouse hook.

Table 13-1. Information Provided by WH_MOUSE and WH_MOUSE_LL Hooks

WH_MOUSE	WH_MOUSE_LL
Cursor coordinates (x,y)	Cursor coordinates (x,y)
Extra information	Extra information
Message ID	Message ID
MSLLHOOKSTRUCT	MOUSEHOOKSTRUCT
Window hwnd	N/A
Hit-test value	N/A
N/A	Mouse wheel delta
N/A	Which XBUTTON