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    <p>I was somewhat surprised in <a href="https://stackoverflow.com/questions/554587/is-there-an-easy-way-to-randomize-a-list-in-vb-net/">this thread</a> how many wrong answers were posted. Just for the sake of others who come up with a solution similar to the one posted by the OP, the following code <em>looks</em> correct:</p> <pre><code>int[] nums = new int[1000]; for (int i = 0; i &lt; nums.Length; i++) { nums[i] = i; } Random r = new Random(); Array.Sort&lt;int&gt;(nums, (x, y) =&gt; r.Next(-1, 2)); foreach(var num in nums) { Console.Write("{0} ", num); } </code></pre> <p>However, the code will throw an exception occasionally, but not always. That's what makes it fun to debug :) If you run it enough times, or execute the sort procedure in a loop 50 or so times, you'll get an error stating:</p> <p><code>IComparer (or the IComparable methods it relies upon) did not return zero when Array.Sort called x. CompareTo(x). x: '0' x's type: 'Int32' The IComparer: ''.</code></p> <p>In other words, the quick sort compared some number <code>x</code> to itself and got a non-zero result. The obvious solution to the code would be write:</p> <pre><code>Array.Sort&lt;int&gt;(nums, (x, y) =&gt; { if (x == y) return 0; else return r.NextDouble() &lt; 0.5 ? 1 : -1; }); </code></pre> <p>But even this doesn't work, because there are occasions where .NET compares 3 numbers against one another which return inconsistent results, such as A > B, B > C, and C > A (oops!). No matter if you use a Guid, GetHashCode, or any other randomly generated input, a solution like the one shown above is still wrong.</p> <hr> <p>With that being said, Fisher-Yates is the standard way of shuffling arrays, so there's no real reason to use IComparer in the first place. Fisher-Yates is O(n) whereas any implementation using IComparer uses a quicksort behinds the scenes which has a time-complexity of O(n log n). There's just no good reason not to use the well-known, efficient, standard algorithm to solve this kind of problem.</p> <p>However, if you really insist on using an IComparer and a rand, then apply your random data <em>before</em> you sort. This requires a projection of the data onto another object so you don't lose your random data:</p> <pre><code>using System; using System.Collections.Generic; using System.Linq; using System.Text; namespace ConsoleApplication1 { class Pair&lt;T, U&gt; { public T Item1 { get; private set; } public U Item2 { get; private set; } public Pair(T item1, U item2) { this.Item1 = item1; this.Item2 = item2; } } class Program { static void Main(string[] args) { Pair&lt;int, double&gt;[] nums = new Pair&lt;int, double&gt;[1000]; Random r = new Random(); for (int i = 0; i &lt; nums.Length; i++) { nums[i] = new Pair&lt;int, double&gt;(i, r.NextDouble()); } Array.Sort&lt;Pair&lt;int, double&gt;&gt;(nums, (x, y) =&gt; x.Item2.CompareTo(y.Item2)); foreach (var item in nums) { Console.Write("{0} ", item.Item1); } Console.ReadKey(true); } } } </code></pre> <p>Or get LINQy with your bad self:</p> <pre><code>Random r = new Random(); var nums = from x in Enumerable.Range(0, 1000) orderby r.NextDouble() select x; </code></pre>
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    1. COI think I put together a pretty good case where you might want to embed correct logic into an IComparer. I know it's not going to be possible to get the same performance as Fisher-Yates, but it ought to be at least possible to get correct logic and reasonable performance.
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