Subclassing and Hooking with Visual Basic

What Is Superclassing?

Superclassing, also known as class cloning, allows us to build upon an existing window class. See Figure 7-1 for an illustration of this technique. The existing class is called the base class. This can be any available class defined by the system or by an application. The base class information, obtained from its WNDCLASSEX structure, is copied into a second WNDCLASSEX structure. This second WNDCLASSEX structure is the beginning of our superclass. Before we have a usable superclass, we must do a couple of things to this new class structure. We must give it a new class name, a new instance handle, and a new window procedure. Also, we must register it with the system. After we do this we have a superclass that we can use to create new specialized windows for our applications.

Figure 7-1. The superclassing technique

The new window procedure we provide for the superclass defines the behavior of windows derived from this class. We can also modify members of the superclass’s’ WNDCLASSEX structure. For example, the style member can be modified to change the window’s border, or the hbrBackground member can be changed to define the window’s default background color. These modifications are left to the developer’s discretion.

When the superclassing application terminates, it should remove all superclasses that it created from memory. It does this through a call to ...