Question

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 

Answer 1

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);     } }