title: Best lookup data structure to store only the keys (Dictionary without value)
body: <p>What is the best data structure in .Net with high performance lookup, like a binary tree implementation, but to store only the keys (string keys) ?</p>
  
  <p>We need only to check if a certain key is present in the collection. Like:</p>
  
  <pre><code>Dictonary&lt;string, object&gt; myKeys;
  myKeys.Add("key1", null);
  myKeys.Add("key2", null);
  // Dozens or hundreds keys
  
  Assert.IsTrue(myKeys.Contains("key1")); 
  </code></pre>