With the exception of some stuff used for windows desktop development, .NET (“dotnet core” is just .NET now) is released under the MIT license. I’m not following how using .NET would be contributing to the “agenda of proprietary software”.
The dotnet cli tools that come with the SDK run just fine cross platforms without Visual Studio. Your Linux distribution probably packages the SDK already, just install and use it.
If you want, you can use C# without .NET by using Unity, mono, or maybe Godot now I think?
Slaves don’t have private ownership of their capital (that is, their own labor)… because someone else does.
Most “free” workers, in terms of capital, own only their own labor.
Capitalists own the majority of the capital–land, equipment, intellectual property, etc.
A system where the workers own the capital (aka the means of production) is socialism.