Subclassing and Hooking with Visual Basic

Subclassing

Subclassing techniques deal with intercepting messages bound for one or more windows or controls. These messages are intercepted before they can reach their destination window. The intercepted message can be left in its original state or modified. Afterward, the message can be sent to its original destination or discarded.

By intercepting messages in this manner, we can have a powerful influence on how the window or control will react to the messages it receives. Consider, for example, right-clicking the Visual Basic (VB) text box control. This action causes a default pop-up menu to be displayed containing the following menu items: Undo, Cut, Copy, Paste, Delete, and Select All. Replacing this menu with one of our own is a fairly simple task using subclassing. Subclassing has many other uses as well, such as:

• Determining when a window is being activated or deactivated and responding to this change

• Responding to new menu items that are manually added to the system menu of a window

• Displaying descriptions of menu items as the mouse moves across them

• Disallowing a user to move or resize a window

• Allowing a user to move or resize a window within specified boundaries

• Determining where the mouse cursor is and responding accordingly

• Modifying the look of a window or control

• Changing the way a combo box operates

• Determining when the display resolution has been changed

• Monitoring the system for a low system-resource condition

• Modifying or disallowing keystrokes sent to a window or ...