title: how to do a dictionary reverse lookup
body: <p>I have a dictionary of type <code>&lt;string, string&gt;</code> and for a particular case, I need to do a reverse lookup. So for instance suppose I have this entry <code>&lt;"SomeString", "ab"&gt;</code> and that I pass in <code>"ab"</code> then I would like to return <code>"SomeString"</code>. 
  Before I embark on a <code>foreach</code> loop over every entry in the dictionary, I was wondering what would be the most efficient way to do this reverse lookup?</p>