Getting key of value of a generic Dictionary?

Question

It's easy to get the value of a key from a .Net 2.0 generic Dictionary:

Dictionary<int, string> greek = new Dictionary<int, string>();
greek.Add(1, "Alpha");
greek.Add(2, "Beta");
string secondGreek = greek[2];  // Beta

But is there a simple way to get the key of a value?

int[] betaKeys = greek.WhatDoIPutHere("Beta");  // expecting single 2

Answer 1

As a twist of the accepted answer (https://stackoverflow.com/a/255638/986160) assuming that the keys will be associated with signle values in the dictionary. Similar to (https://stackoverflow.com/a/255630/986160) but a bit more elegant. The novelty is in that the consuming class can be used as an enumeration alternative (but for strings too) and that the dictionary implements IEnumerable.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Collections;

namespace MyApp.Dictionaries
{

    class BiDictionary<TFirst, TSecond> : IEnumerable
    {
        IDictionary<TFirst, TSecond> firstToSecond = new Dictionary<TFirst, TSecond>();
        IDictionary<TSecond, TFirst> secondToFirst = new Dictionary<TSecond, TFirst>();

        public void Add(TFirst first, TSecond second)
        {
            firstToSecond.Add(first, second);
            secondToFirst.Add(second, first);
        }

        public TSecond this[TFirst first]
        {
            get { return GetByFirst(first); }
        }

        public TFirst this[TSecond second]
        {
            get { return GetBySecond(second); }
        }

        public TSecond GetByFirst(TFirst first)
        {
            return firstToSecond[first];
        }

        public TFirst GetBySecond(TSecond second)
        {
            return secondToFirst[second];
        }

        public IEnumerator GetEnumerator()
        {
            return GetFirstEnumerator();
        }

        public IEnumerator GetFirstEnumerator()
        {
            return firstToSecond.GetEnumerator();
        }

        public IEnumerator GetSecondEnumerator()
        {
            return secondToFirst.GetEnumerator();
        }
    }
}

And as a consuming class you could have

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace MyApp.Dictionaries
{
    class Greek
    {

        public static readonly string Alpha = "Alpha";
        public static readonly string Beta = "Beta";
        public static readonly string Gamma = "Gamma";
        public static readonly string Delta = "Delta";


        private static readonly BiDictionary<int, string> Dictionary = new BiDictionary<int, string>();


        static Greek() {
            Dictionary.Add(1, Alpha);
            Dictionary.Add(2, Beta);
            Dictionary.Add(3, Gamma);
            Dictionary.Add(4, Delta);
        }

        public static string getById(int id){
            return Dictionary.GetByFirst(id);
        }

        public static int getByValue(string value)
        {
            return Dictionary.GetBySecond(value);
        }

    }
}

Answer 2

The "simple" bidirectional dictionary solution proposed here is complex and may be be difficult to understand, maintain or extend. Also the original question asked for "the key for a value", but clearly there could be multiple keys (I've since edited the question). The whole approach is rather suspicious.

Software changes. Writing code that is easy to maintain should be given priority other "clever" complex workarounds. The way to get keys back from values in a dictionary is to loop. A dictionary isn't designed to be bidirectional.

Answer 3

Dictionaries aren't really meant to work like this, because while uniqueness of keys is guaranteed, uniqueness of values isn't. So e.g. if you had

var greek = new Dictionary<int, string> { { 1, "Alpha" }, { 2, "Alpha" } };

What would you expect to get for greek.WhatDoIPutHere("Alpha")?

Therefore you can't expect something like this to be rolled into the framework. You'd need your own method for your own unique uses---do you want to return an array (or IEnumerable<T>)? Do you want to throw an exception if there are multiple keys with the given value? What about if there are none?

Personally I'd go for an enumerable, like so:

IEnumerable<TKey> KeysFromValue<TKey, TValue>(this Dictionary<TKey, TValue> dict, TValue val)
{
    if (dict == null)
    {
        throw new ArgumentNullException("dict");
    }
    return dict.Keys.Where(k => dict[k] == val);
}

var keys = greek.KeysFromValue("Beta");
int exceptionIfNotExactlyOne = greek.KeysFromValue("Beta").Single();

Answer 4

Dictionary<string, string> dic = new Dictionary<string, string>();
dic["A"] = "Ahmed";
dic["B"] = "Boys";

foreach (string mk in dic.Keys)
{
    if(dic[mk] == "Ahmed")
    {
        Console.WriteLine("The key that contains \"Ahmed\" is " + mk);
    }
}

Answer 5

revised: okay to have some kind of find you would need something other than dictionary, since if you think about it dictionary are one way keys. that is, the values might not be unique

that said it looks like you're using c#3.0 so you might not have to resort to looping and could use something like:

var key = (from k in yourDictionary where string.Compare(k.Value, "yourValue", true)  == 0 select k.Key).FirstOrDefault();

Answer 6

Use LINQ to do a reverse Dictionary<K, V> lookup. But keep in mind that the values in your Dictionary<K, V> values may not be distinct.

Demonstration:

using System;
using System.Collections.Generic;
using System.Linq;

class ReverseDictionaryLookupDemo
{
    static void Main()
    {
        var dict = new Dictionary<int, string>();
        dict.Add(4, "Four");
        dict.Add(5, "Five");
        dict.Add(1, "One");
        dict.Add(11, "One"); // duplicate!
        dict.Add(3, "Three");
        dict.Add(2, "Two");
        dict.Add(44, "Four"); // duplicate!

        Console.WriteLine("\n== Enumerating Distinct Values ==");
        foreach (string value in dict.Values.Distinct())
        {
            string valueString =
                String.Join(", ", GetKeysFromValue(dict, value));

            Console.WriteLine("{0} => [{1}]", value, valueString);
        }
    }

    static List<int> GetKeysFromValue(Dictionary<int, string> dict, string value)
    {
        // Use LINQ to do a reverse dictionary lookup.
        // Returns a 'List<T>' to account for the possibility
        // of duplicate values.
        return
            (from item in dict
             where item.Value.Equals(value)
             select item.Key).ToList();
    }
}

Expected Output:

== Enumerating Distinct Values ==
Four => [4, 44]
Five => [5]
One => [1, 11]
Three => [3]
Two => [2]