Designing an ActiveX Control
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When designing an ActiveX control, you start by creating a custom VCL control. This forms the basis of your ActiveX control. For information on creating custom controls, see Overview of Component Creation.
When designing the VCL control, keep in mind that it will be embedded in another application; this control is not an application in itself. For this reason, you probably do not want to use elaborate dialog boxes or other major user-interface components. Your goal is typically to make a simple control that works inside of, and follows the rules of the main application.
In addition, you should make sure that the types for all properties and methods you want your object to expose to clients are Automation-compatible, because the ActiveX control's interface must support IDispatch. The wizards do not add any methods to the wrapper class's interface that have parameters that are not Automation-compatible.
The wizards implement all the necessary ActiveX interfaces required using the COM wrapper class. They also surface all Automation-compatible properties, methods, and events through the wrapper class's default interface. Once a wizard has generated the COM wrapper class and its interface, you can use the Type Library Editor to modify the default interface or augment the wrapper class by implementing additional interfaces.