Mind you, the "you can use this other way as you see fit" idea often isn't practical (like combining GC'ed and non-GC'ed parts). You generally want a whole codebase to be structured according to shared idioms. Otherwise the interfacing cost becomes too high.

I have doubts that you can program easily in a C-style way in C# without adding lots of annotations everywhere in many places. But don't know, maybe I'm wrong, I did a search for a simple C-style arena allocator in C#, and it looked acceptable, it was quite close. The most annoying thing was maybe keyword boilerplate.