How a COM Add-In Is Registered

A COM add-in from the standpoint of the Office application is a COM component registered in a particular place in the registry that implements the IDTExtensibility2 interface defined by Office and Visual Studio. From your standpoint as a C# developer, you are writing a C# class that you will compile into an assembly (DLL). Through .NET's' COM interop support, your C# class can be made to look like a COM component to the Office application. You will have to register your add-in just like any COM component to get the Office application to load it.

The registry settings and interface implementation described in this section are created for you automatically when you create an add-in project in Visual Studio. However, it is still important to understand the anatomy of an add-in should you have to troubleshoot add-in issues.