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copied!<p><a href="http://msdn.microsoft.com/en-us/library/xfhwa508.aspx"><code>Dictionary<K,V></code></a> does <em>not</em> have an ordering. Any perceived order maintenance is by chance (and an artifact of a particular implementation including, but not limited to, bucket selection order and count).</p> <p>These are the approaches (just using the <a href="http://msdn.microsoft.com/en-us/netframework/aa569603.aspx">Base Class Libraries BCL</a>) I know about:</p> <ol> <li><a href="http://msdn.microsoft.com/en-us/library/bb460184.aspx"><code>Lookup<K,V></code></a> <ul> <li>.NET4, immutable, can map keys to multiple values (watch for duplicates during building)</li> </ul></li> <li><a href="http://msdn.microsoft.com/en-us/library/system.collections.specialized.ordereddictionary.aspx"><code>OrderedDictionary</code></a> <ul> <li>Old, non-generic, expected Dictionary performance bounds (other two approaches are <code>O(n)</code> for "get(key)/set(key)")</li> </ul></li> <li><code>List<KeyValuePair<K,V>></code> <ul> <li>.NET2/3 okay, mutable, more legwork, can map keys to multiple values (watch for duplicates in inserts)</li> </ul></li> </ol> <p>Happy coding.</p> <hr> <p>Creating a hash data-structure that maintains insertion order is actually only a slight modification of a standard hash implementation (Ruby hashes now maintain insertion order); however, this was not done in .NET nor, more importantly, is it part of the Dictionary/IDictionary contract.</p>

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