A C# Wrapper of the SteamGameNetworkingSockets(https://github.com/ValveSoftware/GameNetworkingSockets) ,now only support on MacOS. However I have done some changes in the SteamGameNetworkingSockets lib so that I can use the wrapper more easily and conveniently. I have forked the lib from the source repo of ValveSoftware, so you can track my SteamGameNetworkingSockets lib in https://github.com/Salty-Sailor/GameNetworkingSockets
The wrapper has two parts: One is the dll which is written in C++, it's a transport level which has TCP-like API but use UDP protocol to send/recv data(See https://github.com/Salty-Sailor/GameNetworkingSockets/blob/master/README.md) The other is a thin shell written in C# so that the developers could use it in Unity or .Net Core Project.
There is a very tiny send/recv sample under SteamWrapper/Test dir.