Value type + readonly field...

gistfile1.cs

using System;

public struct S {
  int _value;
  public S(int value) { _value = value; }
  public int Value { get { return _value; } }
  public void Mutate() { _value++; }
}

public class C {
           S _s = new S(42);
  readonly S _r = new S(42);

  public static void Main() {
    var c = new C();
    c._s.Mutate();
    c._r.Mutate();
    Console.WriteLine(c._s.Value); // 43
    Console.WriteLine(c._r.Value); // 42
  }
}