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?
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.
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