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 a 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.
In this section
A COM interface that you can implement to load and unload managed VSTO Add-ins in Office applications.