Getting key from value in .Net collection

Question

Hi, I am using a StringDictionary to store key value pairs. I need to use the keys and values interchangeably i.e. I should be able to get the key from the value as in my case the values will be distinct.

Is there a direct way I could do it (without looping)? Or if there is any other collection I could use to achieve this?

Currently I am looping:

public String GetKeyFromValue(string value)
        {
            foreach (KeyValuePair<string, string> kvp in instance)
            { 
                if (String.Equals(kvp.Value, value))
                    return kvp.Key;
            }

            throw new Exception ("Key not found in FilterControlMapping");
        }

Any help is much appreciated. Thanks.

Answer 1

The only way I can think of is two dictionaries.

D1 < Key, Value > D2 < Value, Key >

You would be repeating your data though. But you could search both keys and values this way.

Answer 2

This would also be possible using Dictionary:

Dictionary<string, string> sd = new Dictionary<string, string>();
string sKey = sd.Single(kvp => kvp.Value.Equals("A value")).Key;

Note that the Single method will throw if the value occurs more than once. You could use First or even FirstOrDefault.

Edit: As Jon Skeet commented this is still looping. The links he posted to a linked dictionary provide the solution.

Answer 3

You basically need to use two dictionaries encapsulated into one.

There are already implementations here and here in other Stack Overflow questions.

Answer 4

The best idea is to use Dictionary instead of StringDictionary as its obselete.. you can find out more about this here: http://stackoverflow.com/questions/627716/stringdictionary-vs-dictionarystring-string/627733#627733 regarding retrival you should go with linq internally it is using loops but still its much cleaner way..