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1) What is a COM add-in ?
This Is a type of add-in that can be used to provide additional application-level functionality
A COM add-in implements the IDTExtensibility2 interface
It has been possible to create these since Visual Studio 2002.

2) What is a VSTO add-in ?
This is a type of add-in that can be used to provide additional application-level functionality
A VSTO add-in implements the IStartup interface (which is implemented in the Addins class/interface)
This interface is strongly typed and does not rely on late binding.
It has only been possible to create these since Visual Studio 2005.