If your head is dizzy with the myriad VS services and APIs, from EnvDTE to Shell.Interop, this should clarify a couple things.
First a bit of background:
APIs on EnvDTE (DTE for short, since that’s the entry point service you request from the environment) was originally an API intended to be used by macros. It’s also called the automation API. Most of the time, this is a simplified API that is easier to work with, but which doesn’t expose 100% of what VS is capable of doing. It’s also kind of the “rookie” way of doing VS extensibility (VSX for short), since most hardcore VSX devs sooner or later realize that they need to make the leap to the “serious” APIs.
The “real” VSX APIs virtually always start with IVs, make heavy use of uint, ref/out parameters and HResults. These are the APIs that have been evolving for years and years, and there is a lot of COM baggage. ...