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copied!<p>Okay, here's the multiple bidirectional version:</p> <pre><code>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); } } </code></pre>

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