Getting multiple keys of specified value of a generic Dictionary?

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

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

But trying to get the keys given a value is not as straightforward because there could be multiple keys:

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

Okay, here's the multiple bidirectional version:

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

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

    private static IList<TFirst> EmptyFirstList = new TFirst[0];
    private static IList<TSecond> EmptySecondList = new TSecond[0];

    public void Add(TFirst first, TSecond second)
    {
        IList<TFirst> firsts;
        IList<TSecond> seconds;
        if (!firstToSecond.TryGetValue(first, out seconds))
        {
            seconds = new List<TSecond>();
            firstToSecond[first] = seconds;
        }
        if (!secondToFirst.TryGetValue(second, out firsts))
        {
            firsts = new List<TFirst>();
            secondToFirst[second] = firsts;
        }
        seconds.Add(second);
        firsts.Add(first);
    }

    // Note potential ambiguity using indexers (e.g. mapping from int to int)
    // Hence the methods as well...
    public IList<TSecond> this[TFirst first]
    {
        get { return GetByFirst(first); }
    }

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

    public IList<TSecond> GetByFirst(TFirst first)
    {
        IList<TSecond> list;
        if (!firstToSecond.TryGetValue(first, out list))
        {
            return EmptySecondList;
        }
        return new List<TSecond>(list); // Create a copy for sanity
    }

    public IList<TFirst> GetBySecond(TSecond second)
    {
        IList<TFirst> list;
        if (!secondToFirst.TryGetValue(second, out list))
        {
            return EmptyFirstList;
        }
        return new List<TFirst>(list); // Create a copy for sanity
    }
}

class Test
{
    static void Main()
    {
        BiDictionary<int, string> greek = new BiDictionary<int, string>();
        greek.Add(1, "Alpha");
        greek.Add(2, "Beta");
        greek.Add(5, "Beta");
        ShowEntries(greek, "Alpha");
        ShowEntries(greek, "Beta");
        ShowEntries(greek, "Gamma");
    }

    static void ShowEntries(BiDictionary<int, string> dict, string key)
    {
        IList<int> values = dict[key];
        StringBuilder builder = new StringBuilder();
        foreach (int value in values)
        {
            if (builder.Length != 0)
            {
                builder.Append(", ");
            }
            builder.Append(value);
        }
        Console.WriteLine("{0}: [{1}]", key, builder);
    }
}

As everyone else has said, there's no mapping within a dictionary from value to key.

I've just noticed you wanted to map to from value to multiple keys - I'm leaving this solution here for the single value version, but I'll then add another answer for a multi-entry bidirectional map.

The normal approach to take here is to have two dictionaries - one mapping one way and one the other. Encapsulate them in a separate class, and work out what you want to do when you have duplicate key or value (e.g. throw an exception, overwrite the existing entry, or ignore the new entry). Personally I'd probably go for throwing an exception - it makes the success behaviour easier to define. Something like this:

using System;
using System.Collections.Generic;

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

    public void Add(TFirst first, TSecond second)
    {
        if (firstToSecond.ContainsKey(first) ||
            secondToFirst.ContainsKey(second))
        {
            throw new ArgumentException("Duplicate first or second");
        }
        firstToSecond.Add(first, second);
        secondToFirst.Add(second, first);
    }

    public bool TryGetByFirst(TFirst first, out TSecond second)
    {
        return firstToSecond.TryGetValue(first, out second);
    }

    public bool TryGetBySecond(TSecond second, out TFirst first)
    {
        return secondToFirst.TryGetValue(second, out first);
    }
}

class Test
{
    static void Main()
    {
        BiDictionary<int, string> greek = new BiDictionary<int, string>();
        greek.Add(1, "Alpha");
        greek.Add(2, "Beta");
        int x;
        greek.TryGetBySecond("Beta", out x);
        Console.WriteLine(x);
    }
}

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();

Maybe the easiest way to do it, without Linq, can be to loop over the pairs:

int betaKey; 
foreach (KeyValuePair<int, string> pair in lookup)
{
    if (pair.Value == value)
    {
        betaKey = pair.Key; // Found
        break;
    }
}
betaKey = -1; // Not found

If you had Linq, it could have done easily this way:

int betaKey = greek.SingleOrDefault(x => x.Value == "Beta").Key;

A dictionary doesn't keep an hash of the values, only the keys, so any search over it using a value is going to take at least linear time. Your best bet is to simply iterate over the elements in the dictionary and keep track of the matching keys or switch to a different data structure, perhaps maintain two dictionary mapping key->value and value->List_of_keys. If you do the latter you will trade storage for look up speed. It wouldn't take much to turn @Cybis example into such a data structure.

Getting multiple keys of specified value of a generic Dictionary?

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