One more reason to read specification

rtfm.cs

using System;

namespace RTFM
{
	class Program
	{
		private struct Mutable
		{
			private int x;
			public int Mutate()
			{
				x++;
				return x;
			}
		}

		readonly Mutable m = new Mutable();

		public Program()
		{
			Console.WriteLine("-= Mutating from constructor =-");
			Console.WriteLine(m.Mutate());
			Console.WriteLine(m.Mutate());
			Console.WriteLine(m.Mutate());
		}

		static void Main(string[] args)
		{
			Program t = new Program();
			Console.WriteLine("-= Mutating from Main =-");
			Console.WriteLine(t.m.Mutate());
			Console.WriteLine(t.m.Mutate());
			Console.WriteLine(t.m.Mutate());
		}
	}
}