I was trying to port Cap'n Proto to modern C# as a side project when I was unemployed, since the current implementation years old and new C# features have been released that would make it much nicer to use.

I love the no-copy serialization and object capabilities, but wow, the RPC protocol is incredibly complex, it took me a while to wrap my head around it, and I often had to refer to the C++ implementation to really get it.