Subclassing and Hooking with Visual Basic

Chapter 1. Introduction

Windows is a message-based system. This means that every action you take while using the system creates one or more messages to carry out the action. These messages are passed between objects within the system. These messages also carry with them information that gives the recipient more detail on how to interpret and act upon the message.

Clicking a button control provides a good messaging example. This produces not only the message for the mouse button click, but also a wide array of other messages. These include messages to repaint the button in its depressed state, notification messages to inform other objects of the button’s change in state, messages to determine the state of the mouse cursor, as well as others. Even a simple act such as moving the mouse or pressing a key on the keyboard can produce an astonishing number of messages.

In addition to communicating user actions, Windows also uses messages internally to do housekeeping. Messages need to be sent to update the time and date, to notify other objects of a change in state, and even to notify applications when system resources are exhausted.

The Windows messaging system is the heart of the operating system. As a result, the messaging system is very complex.

Subclassing and the Windows hooking mechanism operate on messages within the messaging system. This makes subclassing and hooking two very powerful techniques. With them, we can manipulate, modify, or even discard messages bound for other objects within the operating system and, in the process, change the way in which the system behaves. As you might already have guessed, a thorough understanding of the messaging system is critical to mastering the techniques of subclassing and hooking.

Along with this power comes responsibility. It is up to the developer to make sure that he or she is using these techniques correctly. Windows is very unforgiving if these techniques are used incorrectly.