.NET is still effectively Windows only
You clearly haven't been keeping up then. .NET Core is cross platform, and the version 5 release will see it become the only version of .NET. Version 5.0 release candidates are available now, though I expect there to be a few more iterations before it is production ready.
Sure, the ".NET Desktop Runtime", including the windows forms GUI assemblies, will remain Windows only. But .NET console applications, that avoid direct system calls, should just work on linux.
The windows desktop application is dying anyway. Many people seem to be building web apps instead. It seems like the .NET team are pushing further in this direction with Blazor. Running the .NET runtime in the browser via web assembly, with a HTML UI. (I'm not saying that's a good idea, just that this is where they are heading)