What do you mean by that? Dotnet5+ was made completely* open source, you can look at it on github [GitHub - dotnet/runtime: .NET is a cross-platform runtime for cloud, mobile, desktop, and IoT apps.](https://github.com/dotnet/runtime) Well yes but no, there are some other alternative usages for .net with C# iirc there are JVM-C# version, C#-Embedded and some more. But yes you are more or less fixed to .net, but from my point thats fine. Everything I ever imagined I would like to do, I can with C# (in a reasonable frame). Btw, on another point, mono is the runtime for C# when running on linux nowadays, again: if i remember correctly.