Is it possible to work with C# and write cool apps , programmes on MacOS ?

You can build native apps for Android, iOS, and Windows by using C# or F# (Visual Basic is not supported at this time). To get started, install Visual Studio 2015, select the Custom option in the installer, and check the box under Cross Platform Mobile Development > C#/.NET (Xamarin). https://msdn.microsoft.com/en-IN/library/dn771552.aspx

No, C# native for win , for Mac u need Swift, but u may run some apps on C# with .net on Linux