title: Bi-directional dictionary?
body: <p>I have two sets of objects where in some cases the first set will be used as keys for the other set, and in other cases the second set will be used as keys for the first. In addition there must be no duplicates in either list.</p>
  
  <p>What would be a good data-structure to use for this? Should I use two Dictionaries for this or are there smarter ways?</p>