Methods with MethodImplOptions.InternalCall
are usually internal framework methods. You cannot call them directly or via reflection (which is more or less the same thing).
It depends on the library, I found something like this within the Word API where it internally uses VB. To access variant Properties you'll have to call a setter method for example
Property = "" <- doesn't work
set_Property("") <- works
Depends on the API you are trying to access I guess, if those successors are implemented
Apart from that, maybe read the Platform Invoke Tutorial