title: Alternative to Dictionary for doing fast key lookups?
body: <p>I run into situations where I need to keep track if I've processed a particular value. In these cases, I use <code>Dictionary(Of TKey, TValue)</code> to keep track of the values that I've processed. Basically, as each value is processed, I insert the processed value into as key into the dictionary. When I want to see if I've processed that value, I use the <code>ContainsKey</code> method to see if the value exists in the collection.</p>
  
  <p>This works well, but I do have to insert something in the value side of the key-value pair. I would just use a <code>List(Of T)</code> but I want the performance of the hash table lookup that Dictionary provides. Is there a data collection in .Net that is more suitable for this purpose?</p>