title: Do I have to apply lock even though accessing concurrent dictionary
body: <p>I use a static concurrent dictionary in web api and it is accessed by across the users.</p>
  
  <p>I am using the methods below: Are these methods all thread safe? Or do I have to add <code>lock()</code> even though it is <code>ConccurentDictionary</code>? </p>
  
  <p>Basically, it is accessed by across the user then it should able to work accordingly as this dictionary contain the collection for each user and it relies on that.</p>
  
  <pre><code>static ConcurrentDictionary&lt;string, SApp&gt; SAppFarm= new ConcurrentDictionary&lt;string, SApp&gt;();    
  
  .TryRemove(_sessionUser,out s);
  .TryAdd(_sessionUser, r);
  .GetOrAdd(sessionUser, application);
  </code></pre>