Subclassing and Hooking with Visual Basic

How WH_GETMESSAGE Works

You use the WH_GETMESSAGE hook to intercept only posted (and not sent) messages. Using this hook, you also can modify a message before the window procedure processes it. These are the two main differences between this hook and the WH_CALLWNDPROC and WH_CALLWNDPROCRET hooks. The WH_CALLWNDPROC and WH_CALLWNDPROCRET hooks only intercept sent messages and cannot modify the messages they intercept.

You can use this 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, 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 provides a thorough discussion of this concept.

Before explaining how this hook operates, I need to discuss both the GetMessage and the PeekMessage functions. GetMessage retrieves a single message from the message queue of the thread in which this function was called. This function is used mostly in a message loop for an application. GetMessage is defined as follows:

Public Declare Function GetMessage Lib "user32" Alias "GetMessageA" _
        (lpMsg As MSG, ByVal hwnd As Long, ByVal wMsgFilterMin As Long, _
        ByVal wMsgFilterMax As Long) As Long

GetMessage has the following parameters:

lpMsg

A Long pointer to an MSG structure that receives the message.

hwnd

The handle of the ...