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    <p>If the amount of possible integers from which you draw is significantly larger (say factor 2) than the amount of integers you want you can simply use a <code>HashSet&lt;T&gt;</code> to check for duplicates.</p> <pre><code>List&lt;int&gt; GetUniqueRandoms(Random random, int count) { List&lt;int&gt; result = new List&lt;int&gt;(count); HashSet&lt;int&gt; set = new HashSet&lt;int&gt;(count); for(int i = 0; i &lt; count; i++) { int num; do { num = random.NextInt(); while(!set.Add(num)); result.Add(num); } return result; } </code></pre> <p>This allocates the collections with the correct capacity to avoid reallocation during growth. Since your collections are large this should be a large improvement.</p> <p>You can also use <code>Distinct</code> a single time:</p> <pre><code>IEnumerable&lt;int&gt; RandomSequence(Random random) { while(true) { yield return random.NextInt(); } } RandomSequence(rand).Distinct().Take(1000000000).ToList(); </code></pre> <p>But with both solutions you need enough memory for a <code>HashSet&lt;int&gt;</code> and a <code>List&lt;int&gt;</code>.</p> <hr> <p>If the amount of possible integers from which you draw is about as large as the amount of integers you want, you can create an array that contains all of them, shuffle them and finally cut off those you're not interested in.</p> <p>You can use <a href="https://stackoverflow.com/questions/1287567/c-is-using-random-and-orderby-a-good-shuffle-algorithm/1287572#1287572">Jon Skeet's shuffle implementation</a>.</p>
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    1. POC# creating a list of random unique integers
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    1. COThat will give you a fixed non-random ordering (by hashcode)
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    2. COWhy? I didn't say I want to iterate over the hashset. I'd only use it for checking for duplicates.
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