Subclassing and Hooking with Visual Basic

How WH_CALLWNDPROCRET Works

Like the WH_CALLWNDPROC hook, the WH_CALLWNDPROCRET hook is used to intercept messages sent through the various SendMessage function types. However, WH_CALLWNDPROCRET intercepts its messages later than WH_CALLWNDPROC does: whereas the latter intercepts messages before they are processed by their window procedure, the former intercepts messages only after the window procedure has processed them. As a result, you once again cannot modify or discard the message intercepted through this hook, in this case because message processing is largely complete.

You can use this hook to monitor sent messages, similar to the WH_CALLWNDPROC hook. However, the WH_CALLWNDPROCRET hook provides extra information that WH_CALLWNDPROC does not. First, it makes available the return value of the call to the SendMessage function. Second, the wParam specifies whether the sent message originates from the same process in which the hook is installed, or from another process. An application could use this hook to listen for a specific message that is broadcast from another application and take some action based on receiving that message.

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 a hook is installed as a system-wide hook, it has to be placed in a DLL. This DLL is injected into every process so that ...