I tried .NET out on the Pi using Mono, years ago when it was relatively new but after the software floating point OS was deprecated. The weird hardware floating point CPU made it pretty much unusable. Even simple things like DateTime objects would fail to work correctly.
Similar issues were in Java IIRC but they had a special build of that. I think Sony had a fixed build of Mono but I haven't tried it. It will be good to have official .NET support outside of Win 10 IoT.