title: Is Contains thread safe in HashSet<T>
body: <p>Looking at the code for <code>Contains</code> in the <code>HashSet&lt;T&gt;</code> class in the .NET source code, I cannot find any reason why <code>Contains</code> is not thread safe?</p>
  
  <p>I am loading a <code>HashSet&lt;T&gt;</code> with values ahead of time, and then checking <code>Contains</code> in a multi threaded .<code>AsParallel()</code> loop.</p>
  
  <p>Is there any reason why this would not be safe.
  I am loath to use <code>ConcurrentDictionary</code> when I don't actually require storing values.</p>