title: Why is it not possible to use both readonly and fixed-size buffers in structs in C# 7.2
body: <p>With the release of C# 7.2 there is now the ability to have <code>readonly</code> structs, which in many cases can improve performance.</p>
  
  <p>For one of my structs I am using a fixed-size byte array to actually hold the data. However when I marked the <code>struct</code> and the byte array field <code>readonly</code>, C# compiler complained that <code>readonly</code> is not valid on the field. Why can't I have both <code>fixed</code> and <code>readonly</code> on a field in a <code>struct</code>?</p>
  
  <pre><code>readonly unsafe struct MyStruct {
    readonly fixed byte _Value[6]; //The modifier 'readonly' is not valid for this item.
  }
  </code></pre>