title: immutable value types
body: <p>I am reading Eric Liperts' blog about <a href="http://ericlippert.com/2008/05/14/mutating-readonly-structs/" rel="nofollow">Mutating Readonly Structs</a> and I see many references here in SO to this blog as an argument why value types must be immutable. 
  But still one thing is not clear, says that when you access value type you always get the copy of it and here is the example :</p>
  
  <pre><code>struct Mutable
  {
      private int x;
      public int Mutate()
      {
          this.x = this.x + 1;
          return this.x;
      }
  }
  
  class Test
  {
      public readonly Mutable m = new Mutable();
      static void Main(string[] args)
      {
          Test t = new Test();
          System.Console.WriteLine(t.m.Mutate());
          System.Console.WriteLine(t.m.Mutate());
          System.Console.WriteLine(t.m.Mutate());
      }
  }
  </code></pre>
  
  <p>And the question is this why when I change the  </p>
  
  <pre><code>public readonly Mutable m = new Mutable();
  </code></pre>
  
  <p>to </p>
  
  <pre><code>public Mutable m = new Mutable();
  </code></pre>
  
  <p>everything starts to work es <em>expected</em>.</p>
  
  <p>Please can you explain more clear why Value Types must be immutable. 
  I know that it is good for thread safety, but in this case same can be applied to reference types.</p>