how to do a dictionary reverse lookup

Question

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

Answer 1

Basically, You can use LINQ and get the Key like this, without reversing anything:

var key = dictionary.FirstOrDefault(x => x.Value == "ab").Key; 

If you really want to reverse your Dictionary, you can use an extension method like this:

public static Dictionary<TValue, TKey> Reverse<TKey, TValue>(this IDictionary<TKey, TValue> source) {      var dictionary = new Dictionary<TValue, TKey>();      foreach (var entry in source)      {          if(!dictionary.ContainsKey(entry.Value))              dictionary.Add(entry.Value, entry.Key);      }      return dictionary; }  

Then you can use it like this:

var reversedDictionary = dictionary.Reverse(); var key = reversedDictionary["ab"]; 

Note: if you have duplicate values then this method will add the first Value and ignore the others.

Answer 2

Use the Linq ToDictionary function:

var reversed = d.ToDictionary(x => x.Value, x => x.Key); 

You can see below that it works, as tested in Linqpad:

var d = new Dictionary<int, string>(); d.Add(1,"one"); d.Add(2,"two"); d.Dump(); //prints it out in linq-pad var reversed = d.ToDictionary(x => x.Value, x => x.Key); reversed.Dump(); //prints it out in linq-pad