I hope you haven't been trying to use Mono or something obscure for this, which unfortunately happens from time to time.

If you have .NET SDK installed (you can get it with apt/dnf install dotnet8 or dotnet-sdk-8.0), you only need the following:

  dotnet new install Avalonia.Templates
  dotnet new avalonia.app
  dotnet run

If you don't like XAML, you can use https://github.com/AvaloniaUI/Avalonia.Markup.Declarative to write declarative SwiftUI-like code. You can also use F# if that's your cup of tea: https://github.com/fsprojects/Avalonia.FuncUI.

If you prefer GTK, there are rich GObject bindings that are a successor to GTK#: https://gircore.github.io/

Here are samples that demonstrate basic GTK4 usage scenarios: https://github.com/gircore/gir.core/tree/main/src/Samples/Gt...

All this should require less than 10 minutes including setup and such.

Lastly, I want to make a disclaimer that you do not need C# Dev Kit extension (which requires an account that annoys many people, including me) for VS Code, only the base C# one, which is what gives you language server, debugger, etc. If you are using VSCodium which cannot use closed-source vsdbg component that the base extension uses, you can replace it with https://github.com/muhammadsammy/free-vscode-csharp which uses open-source debugger from Samsung instead. It can be rough around the edges but works well enough in standard scenarios. Just don't use Debugger.WriteLine over Console. :D