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copied!<p>I don't think so. Looking through Reflector, the implementation of <code>String.Join</code> looks very optimized. It also has the added benefit of knowing the total size of the string to be created in advance, so it doesn't need any reallocation.</p> <p>I have created two test methods to compare them:</p> <pre><code>public static string TestStringJoin(double[][] array) { return String.Join(Environment.NewLine, Array.ConvertAll(array, row => String.Join(",", Array.ConvertAll(row, x => x.ToString())))); } public static string TestStringBuilder(double[][] source) { // based on Marc Gravell's code StringBuilder sb = new StringBuilder(); foreach (var row in source) { if (row.Length > 0) { sb.Append(row[0]); for (int i = 1; i < row.Length; i++) { sb.Append(',').Append(row[i]); } } } return sb.ToString(); } </code></pre> <p>I ran each method 50 times, passing in an array of size <code>[2048][64]</code>. I did this for two arrays; one filled with zeros and another filled with random values. I got the following results on my machine (P4 3.0 GHz, single-core, no HT, running Release mode from CMD):</p> <pre><code>// with zeros: TestStringJoin took 00:00:02.2755280 TestStringBuilder took 00:00:02.3536041 // with random values: TestStringJoin took 00:00:05.6412147 TestStringBuilder took 00:00:05.8394650 </code></pre> <p>Increasing the size of the array to <code>[2048][512]</code>, while decreasing the number of iterations to 10 got me the following results:</p> <pre><code>// with zeros: TestStringJoin took 00:00:03.7146628 TestStringBuilder took 00:00:03.8886978 // with random values: TestStringJoin took 00:00:09.4991765 TestStringBuilder took 00:00:09.3033365 </code></pre> <p>The results are repeatable (almost; with small fluctuations caused by different random values). Apparently <code>String.Join</code> is a little faster most of the time (although by a very small margin).</p> <p>This is the code I used for testing:</p> <pre><code>const int Iterations = 50; const int Rows = 2048; const int Cols = 64; // 512 static void Main() { OptimizeForTesting(); // set process priority to RealTime // test 1: zeros double[][] array = new double[Rows][]; for (int i = 0; i < array.Length; ++i) array[i] = new double[Cols]; CompareMethods(array); // test 2: random values Random random = new Random(); double[] template = new double[Cols]; for (int i = 0; i < template.Length; ++i) template[i] = random.NextDouble(); for (int i = 0; i < array.Length; ++i) array[i] = template; CompareMethods(array); } static void CompareMethods(double[][] array) { Stopwatch stopwatch = Stopwatch.StartNew(); for (int i = 0; i < Iterations; ++i) TestStringJoin(array); stopwatch.Stop(); Console.WriteLine("TestStringJoin took " + stopwatch.Elapsed); stopwatch.Reset(); stopwatch.Start(); for (int i = 0; i < Iterations; ++i) TestStringBuilder(array); stopwatch.Stop(); Console.WriteLine("TestStringBuilder took " + stopwatch.Elapsed); } static void OptimizeForTesting() { Thread.CurrentThread.Priority = ThreadPriority.Highest; Process currentProcess = Process.GetCurrentProcess(); currentProcess.PriorityClass = ProcessPriorityClass.RealTime; if (Environment.ProcessorCount > 1) { // use last core only currentProcess.ProcessorAffinity = new IntPtr(1 << (Environment.ProcessorCount - 1)); } } </code></pre>

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