how to do a dictionary reverse lookup

c#

Solution

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.

Problem

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?

Original source