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copied!<p>Using a StringBuilder to assemble the string will get you a 600 times increase in processing (as it avoids repeated object creation everytime you append to the string.</p> <p>before loop (initialising capacity avoids recreating the backing array in StringBuilder):</p> <pre><code>StringBuilder sb = new StringBuilder(1000000 * ReadLength); </code></pre> <p>in loop:</p> <pre><code>sb.Append(all.Substring(randomPos, ReadLength) + Environment.NewLine); </code></pre> <p>after loop:</p> <pre><code>readString = sb.ToString(); </code></pre> <p>Using a char array instead of a string to extract the values yeilds another 30% improvement as you avoid object creation incurred when calling Substring():</p> <p>before loop:</p> <pre><code>char[] chars = all.ToCharArray(); </code></pre> <p>in loop:</p> <pre><code>sb.Append(chars, randomPos, ReadLength); sb.AppendLine(); </code></pre> <p><strong>Edit (final version which does not use StringBuilder and executes in 300ms):</strong></p> <pre><code>char[] chars = all.ToCharArray(); var iterations = 1000000; char[] results = new char[iterations * (ReadLength + 1)]; GetRandomStrings(len, iterations, ReadLength, chars, results, 0); string s = new string(results); private static void GetRandomStrings(int len, int iterations, int ReadLength, char[] chars, char[] result, int resultIndex) { Random random = new Random(); int i = 0, index = resultIndex; while (i < iterations && len - 100 > 0) //len is 3000000 { var i1 = len - ReadLength; int randomPos = random.Next() % i1; Array.Copy(chars, randomPos, result, index, ReadLength); index += ReadLength; result[index] = Environment.NewLine[0]; index++; i++; } } </code></pre>

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