Subclassing and Hooking with Visual Basic

Description

Both the WH_SYSMSGFILTER and the WH_MSGFILTER hooks allow you to monitor or discard messages dealing with menus, modal dialog boxes, message boxes, and scrollbars. These messages are intercepted before the window procedure or any subclass procedure processes them. As mentioned in Chapter 15, the WH_SYSMSGFILTER hook intercepts messages only belonging to this set of items because they all operate within a modal loop. See Chapter 15 for more information about the modal loop and how it operates.

You cannot use this hook as a thread-specific hook, you must place it in a dynamic link library (DLL) and use it as a system-wide hook. When you install a hook as a system-wide hook, you must place it in a DLL. This DLL is injected into every process so that the hook can operate on all messages in the system. Chapter 3 discusses this in detail.

A big difference between the WH_SYSMSGFILTER hook and the WH_MSGFILTER hook is that you can use the WH_SYSMSGFILTER hook only as a system-wide hook, whereas you can use the WH_MSGFILTER hook as both a system-wide and a thread-specific hook.

Table 16-1 compares the WH_SYSMSGFILTER and WH_MSGFILTER hooks. Looking at this table, we can determine that the WH_SYSMSGFILTER hook essentially contains a subset of the functionality of the WH_MSGFILTER hook. However, note the order in which the system calls these two hooks. WH_SYSMSGFILTER hooks are always called before WH_MSGFILTER hooks. The thing to remember here is that if any of the WH_SYSMSGFILTER ...