C#循环效率比较 Item6 - Prefer 'for' over 'foreach'. [Performance]

Item6 - Prefer 'for' over 'foreach'. [Performance]

The main differences in using the foreach statement are that you do not need to deal with increments and with the end of the loop expression. Moreover, the foreach statement is designed to traverse through the entire collection. One can say that foreach is a private case of for.

In the code snippets below, we can see that both loop blocks produce the same results, only under the hood the foreach hurts the performance. More variables are involved and additional heavy array copy.

The foreach is far more handier to use especially for collections but if your code runs over large collections, prefer using 'for'.

//foreach

Stopwatch watch = new Stopwatch();
watch.Start();
int[] arrayOfInts = new int[50000000];
int sum = 0;
foreach (int i in arrayOfInts) {
sum += i;
}
watch.Stop();
  System.Console.WriteLine(watch.ElapsedMilliseconds + "ms");  //431ms
watch.Reset();

watch.Start();
//for

sum = 0;
for (int i = 0; i < arrayOfInts.Length; i++) {
sum += arrayOfInts[i];
}
watch.Stop();
  System.Console.WriteLine(watch.ElapsedMilliseconds + "ms"); //313ms