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Check Lazarus, it's not c# but Object Pascal, but it's a decent IDE for developing cross-platform GUI programs, it's concept is from Delphi which was from Visual Basic.

Code editor, GUI designer, UI framework, runtime library, debugger, everything is included and it's being developed actively for nearly 2 decades.

At the moment there is Xamarin.Forms which actually can target osx,gdk and uwp or wpf. which means you actually can have a cross plat gui on all platforms, stability differs for each platform and I also did not like the mono runtime that much, a lot of stuff is not as good as it is in the .net core runtime.

https://github.com/aspnet/Blazor

This is the Electron demo using Blazor by Steve Sanderson. https://github.com/SteveSandersonMS/BlazorElectronExperiment...

https://github.com/nventive/Uno

Electron.NET does exactly that https://github.com/ElectronNET/Electron.NET