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POIn .NET, using "foreach" to iterate an instance of IEnumerable<ValueType> will create a copy? So should I prefer to use "for" instead of "foreach"?
 

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  1. POIn .NET, using "foreach" to iterate an instance of IEnumerable<ValueType> will create a copy? So should I prefer to use "for" instead of "foreach"?
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    <p>In .NET, using "foreach" to iterate an instance of IEnumerable will create a copy? So should I prefer to use "for" instead of "foreach"?</p> <p>I wrote some code to testify this:</p> <pre><code>struct ValueTypeWithOneField { private Int64 field1; } struct ValueTypeWithFiveField { private Int64 field1; private Int64 field2; private Int64 field3; private Int64 field4; private Int64 field5; } public class Program { static void Main(string[] args) { Console.WriteLine("one field"); Test&lt;ValueTypeWithOneField&gt;(); Console.WriteLine("-----------"); Console.WriteLine("Five field"); Test&lt;ValueTypeWithFiveField&gt;(); Console.ReadLine(); } static void Test&lt;T&gt;() { var test = new List&lt;T&gt;(); for (int i = 0; i &lt; 5000000; i++) { test.Add(default(T)); } Stopwatch sw = new Stopwatch(); for (int i = 0; i &lt; 5; i++) { sw.Start(); foreach (var item in test) { } sw.Stop(); Console.WriteLine("foreach " + sw.ElapsedMilliseconds); sw.Restart(); for (int j = 0; j &lt; test.Count; j++) { T temp = test[j]; } sw.Stop(); Console.WriteLine("for " + sw.ElapsedMilliseconds); sw.Reset(); } }} </code></pre> <p>And this is the result that I got after I ran the code:</p> <pre><code> one field foreach 68 for 72 foreach 68 for 72 foreach 67 for 72 foreach 64 for 73 foreach 68 for 72 ----------- Five field foreach 272 for 193 foreach 273 for 191 foreach 272 for 190 foreach 271 for 190 foreach 275 for 188 </code></pre> <p>As we can see in the result, "foreach" always takes more time than "for". </p> <p>So should I prefer to use "for" instead of "foreach" when iterating through a generic collection of value type?</p> <p><strong>Note: thanks for the reminder, I edited the code and result. but still, foreach is running slower than for.</strong></p>
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    1. USCui Pengfei 崔鹏飞
    1. USCui Pengfei 崔鹏飞
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    1. COI don't think your test is right. The foreach will assign a value to **item**, but the for loop doesn't assign anything. What happens if you actually do an assignment in the for loop: var k = test[j];
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    2. COI think, to get this test more accurate, you should do someting simple with the actual element. Like `var l = item + 1;` / `var l = test[i] + 1`. Furthermore you have to call the GetEnumerator function in the foreach loop, while for loop only counts without touching the lists.
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    3. CO@Tokk: `item++` would not be allowed. You can't modify the iterator variable. The test needs equivalent actions, which means the `for` block needs to retrieve the value.
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      1. USAdam Robinson
 

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