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    <p>Provided the memory footprint is tolerable, I would suggest a <code>Hashset&lt;string&gt;</code> for the strings, and a queue to store a <code>Tuple&lt;DateTime, String&gt;</code>. Something like:</p> <pre><code>Hashset&lt;string&gt; Strings = new HashSet&lt;string&gt;(); Queue&lt;Tuple&lt;DateTime, String&gt;&gt; Expirations = new Queue&lt;Tuple&lt;DateTime, String&gt;&gt;(); </code></pre> <p>Now, when a string comes in:</p> <pre><code>if (Strings.Add(s)) { // string is new. process it. // and add it to the expiration queue Expirations.Enqueue(new Tuple&lt;DateTime, String&gt;(DateTime.Now + ExpireTime, s)); } </code></pre> <p>And, somewhere you'll have to check for the expirations. Perhaps every time you get a new string, you do this:</p> <pre><code>while (Expirations.Count &gt; 0 &amp;&amp; Expirations.Peek().Item1 &lt; DateTime.Now) { var e = Expirations.Dequeue(); Strings.Remove(e.Item2); } </code></pre> <p>It'd be hard to beat the performance of <code>Hashset</code> here. Granted, you're storing the strings, but that's going to be the only way to guarantee no false positives.</p> <p>You might also consider using a time stamp other than <code>DateTime.Now</code>. What I typically do is start a <code>Stopwatch</code> when the program starts, and then use the <code>ElapsedMilliseconds</code> value. That avoids potential problems that occur during Daylight Saving Time changes, when the system automatically updates the clock (using NTP), or when the user changes the date/time.</p> <p>Whether the above solution works for you is going to depend on whether you can stand the memory hit of storing the strings.</p> <p>Added after "Additional information" was posted:</p> <p>If this will be accessed by multiple threads, I'd suggest using <code>ConcurrentDictionary</code> rather than <code>Hashset</code>, and <code>BlockingCollection</code> rather than <code>Queue</code>. Or, you could use <code>lock</code> to synchronize access to the non-concurrent data structures.</p> <p>If it's true that 99% of the strings will not be duplicate, then you'll almost certainly need an expiration queue that can remove things from the dictionary.</p>
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    1. PODatastructure choices for highspeed and memory efficient detection of duplicate of strings
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    1. USJim Mischel
    1. USJim Mischel
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    1. COWhy would you use the queue instead of a dictionary?
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      1. USJonathan Holland
    2. COThis strikes me as overkill. A `Dictionary<string, DateTime>` would be a far simpler approach. A single data structure and a single `if` boolean expression would be all that's needed to be the gatekeeper.
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      1. USAnthony Pegram
    3. CO@JonathanHolland: because you want the expirations in time order. If you use the dictionary, then you have to iterate over the entire dictionary to find the items that need to expire. With the queue, you always know that the next item to expire is at the head of the queue. Assuming, of course, that all strings have the same expiration times (i.e. always expire a string after 5 minutes or whatever). If different strings have different expiration periods, you'll have to use a priority queue.
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      1. USJim Mischel
 

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