Subclassing and Hooking with Visual Basic

Subclassing a UserControl from Within a VB-Created ActiveX Control

The UserControl is the base on which you create your control. You can subclass the UserControl while creating your own ActiveX control. For example, you might need to watch for specific mouse or keyboard events that are directed to your control. To watch for these and any other messages, you need to subclass the UserControl.

Subclassing a UserControl when only one control is on the form at any one time is easy. In fact, it is not much different from subclassing any other window. The problem occurs when more than one of the subclassed controls exists on a form at any one time.

To see what I mean, let’s examine a little more of how subclassing works with a VB ActiveX control. The subclass procedure must exist in a .bas module, and one instance of this module exists for every control that is instantiated. However, one instance of a UserControl module is created for each instance of the control. When the control is subclassed, a message will first be sent to the only function in the .bas module that is acting as the subclass procedure for all instances of the control. This procedure must then know which instance of the UserControl module to forward the message to so that it can be processed. After the message is processed, it must be sent to the original window procedure of that instance of the control. If the message is not forwarded to the correct original window procedure, all instances of the control might seem ...