Unmanaged API Reference (Office Development in Visual Studio)
Starting with the 2007 Microsoft Office system, Office applications use the IManagedAddin Interface interface to call into an VSTO Add-in loader component that is included with the Visual Studio Tools for Office runtime. This component is used to help load managed VSTO Add-ins. You can create your own VSTO Add-in loader component by implementing this interface.
A COM interface that you can implement to load and unload managed VSTO Add-ins in Office applications.