I've been looking at the VSTO libraries, mostly for the fun of it, and I noticed that there's something called Inspector as well as InspectorClass.
What's the difference, why is it there and how can I put it to use? (NB I'm not looking for an answer on how to code using those classes but rather what the rationale behind that certain architecture pattern is. Purely academical curiosity.)
InspectorClass is a coclass (concrete implementation) of the Inspector COM interface (e.g. it is a COM object with metadata + code). An Inspector can be instantiated directly even though it exists purely as an interface definition (it actually instantiates InspectorClassbehind the scenes).
InspectorClass can be instantiated since it represents a concrete class instance, although as VSTO added support for embedding interop types in .NET 4 - support for the *Class usage is no longer used and serves to exist more or less for backwards compatibility.
All this to say is that you should now be using Inspector and not InspectorClass which can contain executable code. From MSDN blogs:
...it is safe to embed metadata but not anything that can potentially contain executable code (class types contain metadata AND code while interfaces only contain metadata)
From this statement, you can deduce that InspectorClass contains executable code, while Inspector does not - it is purely an interface (metadata). This means that embedding interop types does not allow support for the *Class implementations.