Windows is a message-based system. Every action a user requests creates one or more messages to carry out the action. These messages are passed between objects and carry with them information that gives the recipient more detail on how to interpret and act upon the message. With Subclassing and the Windows hooking mechanism ( hooks ), a developer can manipulate, modify, or even discard messages bound for other objects within the operating system, in the process changing the way the system behaves. What kinds of results can developers achieve using the power of subclassing and hooking? Here are just a few of the possibilities: determine when a window is being activated or deactivated and respond to this change; display descriptions of menu items as the mouse moves across them; disallow a user to move or resize a window; determine where the mouse cursor is and respond accordingly; determine when the display resolution has been changed; monitor the system for a low system resource condition; modify or disallow keystrokes sent to a window or a control; create an automated testing application; and determine when an application is idle. Along with this power comes responsibility; Windows is very unforgiving if subclassing and hooking are used incorrectly. This text demonstrates the various techniques for intercepting messages bound for one or more windows or controls: the intercepted message can be left in its original state or modified; afterwards, the message can be sent to its original destination or discarded.
Stephen Teilhet earned a degree in electrical engineering but soon afterwards began writing software for the Windows platform. For the last eight years, he has worked for several consulting firms on a wide range of projects, specializing in Visual Basic, Visual C++, MTS, COM, MSMQ, and SQL Server. Stephen currently works for Compuware Numega Labs in Nashua, New Hampshire, where he is immersed in the Microsoft .NET technologies.