Add the correct interface first in Adding an MFC Class from an ActiveX Control wizard.

In good old times aka VC6, if you want to use an ActiveX in MFC, the Components and Controls Gallery would generate all the properties and methods of an ActiveX and pull dependent types and create wrapper classes for them as well. And it was good. It magically knows the main class of the Windows Media Player ActiveX is CWMPPlayer4 and that GetCurrentMedia method returns CWMPMedia.

Welcome to the modern Visual C++. Now comes a new wizard that gives you options to choose interfaces to import from. At the same time you got to decide if you really need that many types from an ActiveX.  If you went the unfortunate path of Adding a Member Variable wizard in the dialog editor and somehow got a COCX1 class back, you don’t have the luxury to choose, and you end up with an CWnd wrapper class that does not really wrap up the properties and methods of the ActiveX. It is still good for early binding (like querying IWMPPlayer4 from the result of CWnd::GetControlUnknown) but if you plan to use late binding and MFC’s OLE support (hello COleException!),  you probably want to delete the COCX1 files and start over with the Adding a Class from an ActiveX Control wizard.

Good thing about the new Adding a Class from an ActiveX Control wizard is that most of time we don’t care about the majority of the types in the ActiveX’s type library. We are not going to call most of the ActiveX’s functionality anyways, so selecting what we would use makes sense. Bad thing is that now we don’t know the return type of a method from the generated classes if the return type is LPDISPATCH, lead to guesswork or fire up OLEView to look up the types in the ActiveX’s type library).

Now comes the worst part of the new wizard. The wizard somehow lost its ability to find the correct ActiveX control class and opt to use the first selected interface to generate a CWnd wrapper.  If you naturally click on the >> button to select all interfaces in the Windows Media Player ActiveX at once, you get a CWMPCdromCollection derived from CWnd. That  won’t work as the ActiveX does not implement the IWMPCdromCollection interfaces (undefined behavior can happen if you call the generated methods), and certainly the WMPCdromCollection class isn’t an insertable ActiveX.

Fortunately it is easy to avoid this issue if you add the ActiveX’s main interface first (e.g. IWMPPlayer4 for the Windows Media Player control) before adding the rest of interfaces, or correct the generated code for the first interface. That is, swap the CWnd and COleDispatchDriver code between the wrong class and the correct class for the main interface. It takes some time to find the correct class for the ActiveX control, however.

Side note: For code cleanness, you probably want to move the InvokeHelper code in the headers to cpp files like VC6 did. A good C++ refactoring tool can help that tremendously.

About Sheng Jiang 蒋晟

Microsoft MVP in Visual C , 2004- Forum moderator of the Visual C and .Net forums on CSDN Forum moderator of Chinese forums on Microsoft's MSDN forums
This entry was posted in enmsdn, MFC, Microsoft, Microsoft Foundation Class Library, Visual C++, Visual Studio and tagged , , , . Bookmark the permalink.

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