Interesting. Its a long time since Ive done anything in C# so Im well out of date on its latest goings on. I dont think C# despite its syntax similarity to Java ever seemed to catch on like MS thought it would. I note they say Roslyn is open source, but presumably Visual C# you still have to pay for?, which iirc used to be 'the' preferred choice of compilers for Windows. The freebie versions used to be very cut down and not much more than notepad++. Not used Mono so wouldnt know how feature rich it was.
10yrs ago one of our lecturers swore .NET would be the biggest thing ever which is why we were thrown into it. I dont think .NET seems to have grown as big nor as fast as M$ hoped. Python, C++ & Java still seem to lead the way. Perhaps this may breath some new life into c#?