Conclusion

ActiveX Controls are one of the most widely used applications of COM in the real world today. To summarize, ActiveX controls are just COM objects that happen to implement a number of standard interfaces that environments like Visual C++ and Visual Basic understand how to use. These interfaces deal with rendering, persistence, and events, allowing you to drop these components into the aforementioned programming environments and use them right away.

In the past, MFC was the only practical way to implement ActiveX Controls. However, these days ATL provides a reasonable way of implementing ActiveX Controls, provided you're willing to follow ATL's rules. For example, if you buy into the ATL architecture for writing controls, you'll have to dip down into Windows and start working with window handles and device context handles in their raw forms. However, the tradeoff is often worthwhile, because ATL provides more flexibility when developing ActiveX controls. For example, dual interfaces are free when using ATL, whereas they're a real pain to implement in MFC.