title: Dictionary Keys.Contains vs. ContainsKey: are they functionally equivalent?
body: <p>I am curious to know if these two are functionally equivalent in all cases.</p>
  
  <p>Is it possible that by changing the dictionary's default comparator that these two would be functionally different?</p>
  
  <p>Also, isn't <code>Keys.Contains</code> almost guaranteed to be slower?</p>