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    <p>Okay, here's the multiple bidirectional version:</p> <pre><code>using System; using System.Collections.Generic; using System.Text; class BiDictionary&lt;TFirst, TSecond&gt; { IDictionary&lt;TFirst, IList&lt;TSecond&gt;&gt; firstToSecond = new Dictionary&lt;TFirst, IList&lt;TSecond&gt;&gt;(); IDictionary&lt;TSecond, IList&lt;TFirst&gt;&gt; secondToFirst = new Dictionary&lt;TSecond, IList&lt;TFirst&gt;&gt;(); private static IList&lt;TFirst&gt; EmptyFirstList = new TFirst[0]; private static IList&lt;TSecond&gt; EmptySecondList = new TSecond[0]; public void Add(TFirst first, TSecond second) { IList&lt;TFirst&gt; firsts; IList&lt;TSecond&gt; seconds; if (!firstToSecond.TryGetValue(first, out seconds)) { seconds = new List&lt;TSecond&gt;(); firstToSecond[first] = seconds; } if (!secondToFirst.TryGetValue(second, out firsts)) { firsts = new List&lt;TFirst&gt;(); 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&lt;TSecond&gt; this[TFirst first] { get { return GetByFirst(first); } } public IList&lt;TFirst&gt; this[TSecond second] { get { return GetBySecond(second); } } public IList&lt;TSecond&gt; GetByFirst(TFirst first) { IList&lt;TSecond&gt; list; if (!firstToSecond.TryGetValue(first, out list)) { return EmptySecondList; } return new List&lt;TSecond&gt;(list); // Create a copy for sanity } public IList&lt;TFirst&gt; GetBySecond(TSecond second) { IList&lt;TFirst&gt; list; if (!secondToFirst.TryGetValue(second, out list)) { return EmptyFirstList; } return new List&lt;TFirst&gt;(list); // Create a copy for sanity } } class Test { static void Main() { BiDictionary&lt;int, string&gt; greek = new BiDictionary&lt;int, string&gt;(); 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&lt;int, string&gt; dict, string key) { IList&lt;int&gt; 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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    1. POGetting key of value of a generic Dictionary?
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    1. USJon Skeet
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    1. POGetting key of value of a generic Dictionary?
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    1. COFrom what I read in msdn, shouldn't this be a BiLookup instead of a BiDictionary? Not that it is important or anything, just curious to if I understand things correctly here...
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    2. COAlso, I used GetByFirst and got back the EmptySecondList, added some things to it and then called GetByFirst again, wouldn't I get a list with some things in it and not an empty list then?
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    3. CO@Svish: No, because when you tried to add to the list it would throw an exception (you can't add to an array). And yes, BiLookup would probably be a better name.
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