mono/mcs/class/System.Core/System/Util.cs

namespace System
{
	internal static class MonoUtil
	{
		public readonly static bool IsUnix;
		
		static MonoUtil ()
		{
			int platform = (int) Environment.OSVersion.Platform;
			IsUnix = platform == 4 || platform == 128 || platform == 6;
		}
	}
}