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copied!<p><strong>I have it running in 0.34 seconds</strong> down from 9+ minutes</p> <p>The problem is when comparing the KeyValuePair structs. I worked around it by writing a comparer object, and passing an instance of it to the Dictionary.</p> <p>From what I can determine, the KeyValuePair.GetHashCode() returns the hashcode of it's <code>Key</code> object (in this example the least unique object).</p> <p>As the dictionary adds (and checks existence of) each item, it uses both Equals and GetHashCode functions, but has to rely on the Equals function when the hashcode is less unique.</p> <p>By providing a more unique GetHashCode function, it excerises the Equals function far less often. I also optimised the Equals function to compare the more unique Values before the less unqiue Keys.</p> <p>86,000 * 11 items with 10,000 unique properties runs in 0.34 seconds using the comparer object below (without the comparer object it takes 9 minutes 22 seconds)</p> <p>Hope this helps :)</p> <pre><code> class StringPairComparer : IEqualityComparer<KeyValuePair<string, string>> { public bool Equals(KeyValuePair<string, string> x, KeyValuePair<string, string> y) { return x.Value == y.Value && x.Key == y.Key; } public int GetHashCode(KeyValuePair<string, string> obj) { return (obj.Key + obj.Value).GetHashCode(); } } </code></pre> <p><strong>EDIT</strong>: If it was just one string (instead of a KeyValuePair, where string = Name+Value) it would be approx twice as fast. It's a nice intresting problem, and I have spent <em>faaaaaar too much time on it</em> (I learned quiet a bit though)</p>

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