Getting key from value in .Net collection

Question

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.

possible duplicate of [Getting key of value of a generic Dictionary?](http://stackoverflow.com/questions/255341/getting-key-of-value-of-a-generic-dictionary) — nawfal, Mar 29 '13 at 05:55

score 5 · Answer 1 · edited May 23 '17 at 12:03

5

You basically need to use two dictionaries encapsulated into one.

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

edited May 23 '17 at 12:03

Community

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answered May 15 '09 at 06:03

Jon Skeet

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That would mean double the memory, wouldn't it? – SO User May 15 '09 at 06:22
The memory of what, exactly? You'd still only have the actual *objects* once (assuming we're talking about reference types), because each collection would just have references to the real values. Yes, the space required for the book-keeping etc would be doubled, but that's the price you pay for being able to look up both ways. – Jon Skeet May 15 '09 at 21:09

score 2 · Answer 2 · answered May 15 '09 at 06:00

2

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.

answered May 15 '09 at 06:00

nunespascal

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Onots · Answer 3 · 2009-05-15T06:14:20.113

0

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.

edited May 15 '09 at 06:14

answered May 15 '09 at 06:00

Onots

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That's *effectively* looping though, i.e. the complexity is O(n) instead of the desirable O(1). – Jon Skeet May 15 '09 at 06:07
Can I use => operator in 2.0? – SO User May 15 '09 at 06:21
No, you can't use the lambda expression. See http://stackoverflow.com/questions/556425/predicate-delegates-in-c on how to achieve the same in 2.0 Though do note Jon Skeet's comment. This will still loop! – Onots May 15 '09 at 10:19

score 0 · Answer 4 · edited May 23 '17 at 12:13

0

The best idea is to use Dictionary instead of StringDictionary as its obselete.. you can find out more about this here: StringDictionary vs Dictionary<string, string> regarding retrival you should go with linq internally it is using loops but still its much cleaner way..

edited May 23 '17 at 12:13

Community

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answered May 15 '09 at 06:06

Usman Masood

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Hey thanks for the info. I had actually migrated from Dictionary to StringDictionary. Time to revert back :) – SO User May 15 '09 at 06:24

Getting key from value in .Net collection

4 Answers4