title: Do not declare read only mutable reference types - why not?
body: <p>I have been reading <a href="https://stackoverflow.com/questions/2274412/immutable-readonly-reference-types-fxcop-violation-do-not-declare-read-only-mu">this question</a> and a few other answers and whilst I get the difference between changing the reference and changing the state of the current instance I'm not certain why this means that I shouldn't mark it readonly.  Is this because marking something as readonly tells the compiler something special about the instance and so it is able to then treat it as thread safe when it actually might not be?</p>
  
  <p>Presumably there are situations where I don't want the instance to be able to be changed, but don't mind if the state of the instance is changed (singleton maybe.  /me prepares for flames)  What are the consequences for marking the instance readonly  if I want this?</p>