COM Shared Add-in (Part 7)

COM Shim to Produce Managed COM Add-in

The shim is a Microsoft Visual C++ Active Template Library (ATL) COM DLL. It exposes a COM-creatable class that acts as a proxy to the real managed extension class. The COM Shim Wizard registers the CLSID and ProgId for this class in the registry. In the case of COM add-ins and smart tags, the wizard also registers this class against all target Microsoft Office applications that you choose. When the host Office application starts, it checks the registry to see which add-ins and smart tags to load, and then uses standard COM object creation to instantiate the registered proxy class. This action loads the COM shim DLL that is registered to proxy one of the add-ins or smart tags.

When the COM shim is loaded and the proxy class is created, the proxy class in turn creates an instance of a second class: the CLR loader class. The CLR loader object loads the Microsoft .NET Framework common language runtime (CLR). It then creates an application domain and loads the managed extension assembly into the new application domain. It creates an instance of the managed extension class that implements the target interface (that is, IDTExtensibility2, ISmartTagRecognizer and/or ISmartTagAction, or IRtdServer) and caches the interface pointer. Subsequently, any calls from Office into this interface are handled first by the proxy class. The proxy class passes those calls on through the cached pointer to the actual extension class in the managed extension assembly.