[j_s]

Apparently this is a hot topic.

The options I'm aware of are:

https://github.com/picoe/Eto (OP - BSD)

https://github.com/AvaloniaUI/Avalonia (MIT)

Currently only with a preview for Mac, Xamarin.Forms is roadmapped for desktop platforms "Q3 2017": https://blog.xamarin.com/glimpse-future-xamarin-forms-3-0/

Licensing for Xamarin Forms was funky for a while (required paying for Xamarin prior to MS acquisition) but appears to be MIT now.

[johnhattan]

And for games, there's MonoGame.

http://www.monogame.net/

It's based on the old XNA framework. At least one successful commercial game (Fez) was written with it.

[pjmlp]

Quite a few successful games were written in it, before Unity changed their license model.

Bastion was another one that comes to my mind.

As additional remark, indies have been doing games in C# since the Managed DirectX days.

[zerr]

Also, Unity is more a tool for designers, while MonoGame or LibGDX are more coding oriented libraries.

[pjmlp]

Kind of agree, but if you look at the curriculum of universities teaching game development degrees, they have all moved into Unity/Unreal, plus the console SDKs.

Also the love they are getting from MS and Google for VR/AR.

[geniium]

This is a hot topic? Not so sure.

[j_s]

Either that or someone figured out how to beat HN's vote ring detections to hit #4+ on HN for 2 hours, no?

http://hnrankings.info/14409553/