nmaillet is correct you can develop "Mono" applications on a Mac using Monodevelop, I recently installed it on my iMac without issue. I also develop on windows using Visual Studio. I prefer Visual Studio as it has intellisense and good addins (#reSharper 7.1) that help greatly. But if you are just looking to learn C# development go with MonoDevelop, it'll get you started wihtout costing you anything or requiring new hardware.