The execution platform can be detected by using the System.Environment.OSVersion.Platform value. However correctly detecting Unix platforms, in every cases, requires a little more work. The first versions of the framework (1.0 and 1.1) didn't include any PlatformID value for Unix, so Mono used the value 128. The newer framework 2.0 added Unix to the PlatformID enum but, sadly, with a different value: 4 and newer versions of .NET distinguished between Unix and MacOS X, introducing yet another value 6 for MacOS X. using System; class Program {} See Mono FAQ - How to detect the execution platform. }