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    <p>The reason why is there are several forms of overhead in the foreach version that are not present in the for loop</p> <ul> <li>Use of an IDisposable. </li> <li>An additional method call for every element. Each element must be accessed under the hood by using <code>IEnumerator&lt;T&gt;.Current</code> which is a method call. Because it's on an interface it cannot be inlined. This means N method calls where N is the number of elements in the enumeration. The for loop just uses and indexer</li> <li>In a foreach loop all calls go through an interface. In general this a bit slower than through a concrete type</li> </ul> <p>Please note that the things I listed above are <strong>not</strong> necessarily huge costs. They are typically very small costs that can contribute to a small performance difference. </p> <p>Also note, as Mehrdad pointed out, the compilers and JIT may choose to optimize a foreach loop for certain known data structures such as an array. The end result may just be a for loop. </p> <p>Note: Your performance benchmark in general needs a bit more work to be accurate. </p> <ul> <li>You should use a StopWatch instead of DateTime. It is much more accurate for performance benchmarks. </li> <li>You should perform the test many times not just once</li> <li>You need to do a dummy run on each loop to eliminate the problems that come with JITing a method the first time. This probably isn't an issue when all of the code is in the same method but it doesn't hurt.</li> <li>You need to use more than just 4 values in the list. Try 40,000 instead. </li> </ul>
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    1. POWhy does the second for loop always execute faster than the first one?
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    1. COI'm sorry I wasn't clear. If I put the foreach loop second then it will execute faster then the for loop.
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    2. CO@JaredPar: `foreach` loop for arrays are optimized by the compiler.
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      1. USMehrdad Afshari
    3. COAnother to add to the list is to change the thread priority to something higher to minimise the interference of other threads running on the same processor.
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