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copied!<p>For primitive types (including bytes), use <code>System.Buffer.BlockCopy</code> instead of <code>System.Array.Copy</code>. It's faster.</p> <p>I timed each of the suggested methods in a loop executed 1 million times using 3 arrays of 10 bytes each. Here are the results:</p> <ol> <li>New Byte Array using <code>System.Array.Copy</code> - 0.2187556 seconds</li> <li>New Byte Array using <code>System.Buffer.BlockCopy</code> - 0.1406286 seconds</li> <li>IEnumerable<byte> using C# yield operator - 0.0781270 seconds</li> <li>IEnumerable<byte> using LINQ's Concat<> - 0.0781270 seconds</li> </ol> <p>I increased the size of each array to 100 elements and re-ran the test:</p> <ol> <li>New Byte Array using <code>System.Array.Copy</code> - 0.2812554 seconds</li> <li>New Byte Array using <code>System.Buffer.BlockCopy</code> - 0.2500048 seconds</li> <li>IEnumerable<byte> using C# yield operator - 0.0625012 seconds</li> <li>IEnumerable<byte> using LINQ's Concat<> - 0.0781265 seconds</li> </ol> <p>I increased the size of each array to 1000 elements and re-ran the test:</p> <ol> <li>New Byte Array using <code>System.Array.Copy</code> - 1.0781457 seconds</li> <li>New Byte Array using <code>System.Buffer.BlockCopy</code> - 1.0156445 seconds</li> <li>IEnumerable<byte> using C# yield operator - 0.0625012 seconds</li> <li>IEnumerable<byte> using LINQ's Concat<> - 0.0781265 seconds</li> </ol> <p>Finally, I increased the size of each array to 1 million elements and re-ran the test, executing each loop <strong>only</strong> 4000 times:</p> <ol> <li>New Byte Array using <code>System.Array.Copy</code> - 13.4533833 seconds</li> <li>New Byte Array using <code>System.Buffer.BlockCopy</code> - 13.1096267 seconds</li> <li>IEnumerable<byte> using C# yield operator - 0 seconds</li> <li>IEnumerable<byte> using LINQ's Concat<> - 0 seconds</li> </ol> <p>So, if you need a new byte array, use</p> <pre><code>byte[] rv = new byte[a1.Length + a2.Length + a3.Length]; System.Buffer.BlockCopy(a1, 0, rv, 0, a1.Length); System.Buffer.BlockCopy(a2, 0, rv, a1.Length, a2.Length); System.Buffer.BlockCopy(a3, 0, rv, a1.Length + a2.Length, a3.Length); </code></pre> <p>But, if you can use an <code>IEnumerable<byte></code>, <strong><em>DEFINITELY</em></strong> prefer LINQ's Concat<> method. It's only slightly slower than the C# yield operator, but is more concise and more elegant.</p> <pre><code>IEnumerable<byte> rv = a1.Concat(a2).Concat(a3); </code></pre> <p>If you have an arbitrary number of arrays and are using .NET 3.5, you can make the <code>System.Buffer.BlockCopy</code> solution more generic like this:</p> <pre><code>private byte[] Combine(params byte[][] arrays) { byte[] rv = new byte[arrays.Sum(a => a.Length)]; int offset = 0; foreach (byte[] array in arrays) { System.Buffer.BlockCopy(array, 0, rv, offset, array.Length); offset += array.Length; } return rv; } </code></pre> <p>*Note: The above block requires you adding the following namespace at the the top for it to work.</p> <pre><code>using System.Linq; </code></pre> <p>To Jon Skeet's point regarding iteration of the subsequent data structures (byte array vs. IEnumerable<byte>), I re-ran the last timing test (1 million elements, 4000 iterations), adding a loop that iterates over the full array with each pass:</p> <ol> <li>New Byte Array using <code>System.Array.Copy</code> - 78.20550510 seconds</li> <li>New Byte Array using <code>System.Buffer.BlockCopy</code> - 77.89261900 seconds</li> <li>IEnumerable<byte> using C# yield operator - 551.7150161 seconds</li> <li>IEnumerable<byte> using LINQ's Concat<> - 448.1804799 seconds</li> </ol> <p>The point is, it is <strong><em>VERY</em></strong> important to understand the efficiency of both the creation <em>and the usage</em> of the resulting data structure. Simply focusing on the efficiency of the creation may overlook the inefficiency associated with the usage. Kudos, Jon.</p>

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