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    <p>dan's answer will work and is a correct way to use <code>Parallel.For</code>, but I went through the trouble of profiling the code and I think you'll find that parallelising this won't make it any faster. Each <code>Parellel.For</code> makes several new threads, usually more than three, so with 3 nested <code>Parellel.For</code>s you will have have <em>at least</em> 3^3 (27) threads, which is more way more than the number of logical processors on any machine. The extra threads will if anything add an overhead and slow it down.</p> <p>So why not just have one <code>Parallel.For</code> and 2 ordinary <code>for</code> loops - this will mean that there are around 3-4 threads which will work great on a dual or quad core machine. Like this method:</p> <pre><code>static void Test2(int[] a) { int N = a.Length; int total = 0; Object locker = new object(); Parallel.For(0, N, i =&gt; { for (int j = i + 1; j &lt; N; ++j) for (int k = j + 1; k &lt; N; ++k) if (a[i] + a[j] + a[k] == 30) lock(locker) total++; }); } </code></pre> <p>Take the following code used to profile both methods:</p> <pre><code>class Program { static void Main(string[] args) { Random r = new Random(); int[] arr = new int[100]; arr = arr.Select(i =&gt; r.Next(-30, 30)).ToArray(); Profile(Test0, arr, 20); Profile(Test1, arr, 20); Profile(Test2, arr, 20); Console.WriteLine("Test0: {0} ms", Profile(Test0, arr, 100).TotalMilliseconds); Console.WriteLine("Test1: {0} ms", Profile(Test1, arr, 100).TotalMilliseconds); Console.WriteLine("Test2: {0} ms", Profile(Test2, arr, 100).TotalMilliseconds); Console.ReadLine(); } static void Test0(int[] a) { int N = a.Length; int total = 0; for (int i = 0; i &lt; N; ++i) for (int j = i + 1; j &lt; N; ++j) for (int k = j + 1; k &lt; N; ++k) if (a[i] + a[j] + a[k] == 30) total++; } static void Test1(int[] a) { int N = a.Length; int total = 0; Object locker = new object(); Parallel.For(0, N, i =&gt; Parallel.For(i+1, N, j =&gt; Parallel.For(j+1, N, k =&gt; { if (a[i] + a[j] + a[k] == 30) lock(locker) total++; }))); } static void Test2(int[] a) { int N = a.Length; int total = 0; Object locker = new object(); Parallel.For(0, N, i =&gt; { for (int j = i + 1; j &lt; N; ++j) for (int k = j + 1; k &lt; N; ++k) if (a[i] + a[j] + a[k] == 30) lock(locker) total++; }); } static TimeSpan Profile&lt;T&gt;(Action&lt;T&gt; action, T param, int repeats) { Stopwatch s = new Stopwatch(); for (int i = 0; i &lt; repeats; i++) { s.Start(); action(param); s.Stop(); } return new TimeSpan(s.ElapsedTicks/repeats); } } </code></pre> <p>This yields these results for average execution time for each method: (in Release mode, using .Net 4.0, on a quad core Intel Core i5 machine):</p> <pre><code>Test0: 0.2544 ms Test1: 3.3433 ms Test2: 0.1391 ms </code></pre>
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    1. CO+1 I absolutly agree. In fact I would ask this as a interview question and if they gave me dan's answer I would take points away as it shows they don't understand "why" you should use ParalellFor instaed of just "how" to use ParalellFor
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      1. USScott Chamberlain
    2. COKept meaning to write an answer like this myself, but was too lazy. Well done.
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    3. COGood answer. Test2 is exactly how I would have done this.
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