Mono is developed by Xamarin now owned by Microsoft.
https://en.wikipedia.org/wiki/Mono_(software)
https://en.wikipedia.org/wiki/MonoDevelop
>Mono is a free and open-source project led by Xamarin, a subsidiary of Microsoft (formerly by Novell and originally by Ximian), and the .NET Foundation.
>Mono can be run on many software systems including Android, most Linux distributions, BSD, macOS, Windows, Solaris, and even some game consoles such as PlayStation 3, Wii, and Xbox 360.
>The Mono project has been controversial within the open-source community, as it implements portions of .NET Framework that may be covered by Microsoft patents. Although standardized portions of .NET Framework are covered under Microsoft's "Open Specification Promise"---a covenant stating that Microsoft will not assert its patents against implementations of its specifications under certain conditions—other portions are not, which led to concerns that the Mono project could become the target of patent infringement lawsuits. Following Microsoft's open-sourcing of several core .NET technologies since 2014 and its acquisition of Xamarin in the beginning of 2016, an updated patent promise has been issued for the Mono project
I only worked with C#/.Net once and it was to crack a program and that was pretty easy compared to what I was used to. So in some ways .Net is pretty good.
I remember when Miguel of Gnome was working on the initial Mono implementation while all the patent shit went on with Linux, SCO and Ximian/Novell (who made shady deals with Microsoft and SCO). He was not a very popular fucker in the free software community at that time.