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

Then layman's solution

A function similar to the one below could be written to make such a dictionary:

    public Dictionary<TValue, TKey> Invert(Dictionary<TKey, TValue> dict) {
    Dictionary<TValue, TKey> ret = new Dictionary<TValue, TKey>();
    foreach (var kvp in dict) {ret[kvp.value] = kvp.key;} return ret; }

Answer 2

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);
    }
}

Answer 3

Can't you create a subclass of Dictionary which has that functionality?

    public class MyDict < TKey, TValue > : Dictionary < TKey, TValue >
    {
        private Dictionary < TValue, TKey > _keys;

        public TValue this[TKey key]
        {
            get
            {
                return base[key];
            }
            set 
            { 
                base[key] = value;
                _keys[value] = key;
            }
        }

        public MyDict()
        {
            _keys = new Dictionary < TValue, TKey >();
        }

        public TKey GetKeyFromValue(TValue value)
        {
            return _keys[value];
        }
    }

EDIT: Sorry, didn't get code right first time.

Answer 4

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);
    }
}

Answer 5

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;

Answer 6

As I wanted a full fledged BiDirectional Dictionary (and not only a Map), I added the missing functions to make it an IDictionary compatible class. This is based on the version with unique Key-Value Pairs. Here's the file if desired (Most work was the XMLDoc through):

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

namespace Common
{
    /// <summary>Represents a bidirectional collection of keys and values.</summary>
    /// <typeparam name="TFirst">The type of the keys in the dictionary</typeparam>
    /// <typeparam name="TSecond">The type of the values in the dictionary</typeparam>
    [System.Runtime.InteropServices.ComVisible(false)]
    [System.Diagnostics.DebuggerDisplay("Count = {Count}")]
    //[System.Diagnostics.DebuggerTypeProxy(typeof(System.Collections.Generic.Mscorlib_DictionaryDebugView<,>))]
    //[System.Reflection.DefaultMember("Item")]
    public class BiDictionary<TFirst, TSecond> : Dictionary<TFirst, TSecond>
    {
        IDictionary<TSecond, TFirst> _ValueKey = new Dictionary<TSecond, TFirst>();
        /// <summary> PropertyAccessor for Iterator over KeyValue-Relation </summary>
        public IDictionary<TFirst, TSecond> KeyValue => this;
        /// <summary> PropertyAccessor for Iterator over ValueKey-Relation </summary>
        public IDictionary<TSecond, TFirst> ValueKey => _ValueKey;

        #region Implemented members

        /// <Summary>Gets or sets the value associated with the specified key.</Summary>
        /// <param name="key">The key of the value to get or set.</param>
        /// <Returns>The value associated with the specified key. If the specified key is not found,
        ///      a get operation throws a <see cref="KeyNotFoundException"/>, and
        ///      a set operation creates a new element with the specified key.</Returns>
        /// <exception cref="T:System.ArgumentNullException"><paramref name="key"/> is null.</exception>
        /// <exception cref="T:System.Collections.Generic.KeyNotFoundException">
        /// The property is retrieved and <paramref name="key"/> does not exist in the collection.</exception>
        /// <exception cref="T:System.ArgumentException"> An element with the same key already
        /// exists in the <see cref="ValueKey"/> <see cref="Dictionary&lt;TFirst,TSecond&gt;"/>.</exception>
        public new TSecond this[TFirst key]
        {
            get { return base[key]; }
            set { _ValueKey.Remove(base[key]); base[key] = value; _ValueKey.Add(value, key); }
        }

        /// <Summary>Gets or sets the key associated with the specified value.</Summary>
        /// <param name="val">The value of the key to get or set.</param>
        /// <Returns>The key associated with the specified value. If the specified value is not found,
        ///      a get operation throws a <see cref="KeyNotFoundException"/>, and
        ///      a set operation creates a new element with the specified value.</Returns>
        /// <exception cref="T:System.ArgumentNullException"><paramref name="val"/> is null.</exception>
        /// <exception cref="T:System.Collections.Generic.KeyNotFoundException">
        /// The property is retrieved and <paramref name="val"/> does not exist in the collection.</exception>
        /// <exception cref="T:System.ArgumentException"> An element with the same value already
        /// exists in the <see cref="KeyValue"/> <see cref="Dictionary&lt;TFirst,TSecond&gt;"/>.</exception>
        public TFirst this[TSecond val]
        {
            get { return _ValueKey[val]; }
            set { base.Remove(_ValueKey[val]); _ValueKey[val] = value; base.Add(value, val); }
        }

        /// <Summary>Adds the specified key and value to the dictionary.</Summary>
        /// <param name="key">The key of the element to add.</param>
        /// <param name="value">The value of the element to add.</param>
        /// <exception cref="T:System.ArgumentNullException"><paramref name="key"/> or <paramref name="value"/> is null.</exception>
        /// <exception cref="T:System.ArgumentException">An element with the same key or value already exists in the <see cref="Dictionary&lt;TFirst,TSecond&gt;"/>.</exception>
        public new void Add(TFirst key, TSecond value) {
            base.Add(key, value);
            _ValueKey.Add(value, key);
        }

        /// <Summary>Removes all keys and values from the <see cref="Dictionary&lt;TFirst,TSecond&gt;"/>.</Summary>
        public new void Clear() { base.Clear(); _ValueKey.Clear(); }

        /// <Summary>Determines whether the <see cref="Dictionary&lt;TFirst,TSecond&gt;"/> contains the specified
        ///      KeyValuePair.</Summary>
        /// <param name="item">The KeyValuePair to locate in the <see cref="Dictionary&lt;TFirst,TSecond&gt;"/>.</param>
        /// <Returns>true if the <see cref="Dictionary&lt;TFirst,TSecond&gt;"/> contains an element with
        ///      the specified key which links to the specified value; otherwise, false.</Returns>
        /// <exception cref="T:System.ArgumentNullException"><paramref name="item"/> is null.</exception>
        public bool Contains(KeyValuePair<TFirst, TSecond> item) => base.ContainsKey(item.Key) & _ValueKey.ContainsKey(item.Value);

        /// <Summary>Removes the specified KeyValuePair from the <see cref="Dictionary&lt;TFirst,TSecond&gt;"/>.</Summary>
        /// <param name="item">The KeyValuePair to remove.</param>
        /// <Returns>true if the KeyValuePair is successfully found and removed; otherwise, false. This
        ///      method returns false if <paramref name="item"/> is not found in the <see cref="Dictionary&lt;TFirst,TSecond&gt;"/>.</Returns>
        /// <exception cref="T:System.ArgumentNullException"><paramref name="item"/> is null.</exception>
        public bool Remove(KeyValuePair<TFirst, TSecond> item) => base.Remove(item.Key) & _ValueKey.Remove(item.Value);

        /// <Summary>Removes the value with the specified key from the <see cref="Dictionary&lt;TFirst,TSecond&gt;"/>.</Summary>
        /// <param name="key">The key of the element to remove.</param>
        /// <Returns>true if the element is successfully found and removed; otherwise, false. This
        ///      method returns false if <paramref name="key"/> is not found in the <see cref="Dictionary&lt;TFirst,TSecond&gt;"/>.</Returns>
        /// <exception cref="T:System.ArgumentNullException"><paramref name="key"/> is null.</exception>
        public new bool Remove(TFirst key) => _ValueKey.Remove(base[key]) & base.Remove(key);

        /// <Summary>Gets the key associated with the specified value.</Summary>
        /// <param name="value">The value of the key to get.</param>
        /// <param name="key">When this method returns, contains the key associated with the specified value,
        ///      if the value is found; otherwise, the default value for the type of the key parameter.
        ///      This parameter is passed uninitialized.</param>
        /// <Returns>true if <see cref="ValueKey"/> contains an element with the specified value; 
        ///      otherwise, false.</Returns>
        /// <exception cref="T:System.ArgumentNullException"><paramref name="value"/> is null.</exception>
        public bool TryGetValue(TSecond value, out TFirst key) => _ValueKey.TryGetValue(value, out key);
        #endregion
    }
}