Foreach vs For loop weird results

Question

I am running a simple test on loops to see which is inherently faster because my project will be looping over millions of objects. This test fills a List with 1 millions objects, each having a MyClass.Age property. I loop through each objects and pull out the Age property. I do this in a Foreach loop and a For loop. Can someone please explain the weird results OR show me that I've made a stupid error... Thanks

    static int trial = 1;
    static void Main(string[] args)
    {
        while (Console.ReadKey().Key == ConsoleKey.Enter)
        {
            Run();
            trial++;
        }
    }

    private static void Run()
    {
        Console.WriteLine();
        Console.WriteLine("Trial " + trial.ToString());

        List<MyClass> classes = new List<MyClass>();
        for (int x = 1; x <= 1000000; x++)
        {
            classes.Add(new MyClass(x));
        }

        DateTime startTime = new DateTime();
        DateTime endTime = new DateTime();

        DateTime startTime2 = new DateTime();
        DateTime endTime2 = new DateTime();

        startTime = DateTime.Now;
        foreach (MyClass c in classes)
        {
            int age = c.Age;
        }
        endTime = DateTime.Now;
        TimeSpan span = endTime - startTime;
        Console.WriteLine("ForEach Time:  " + span.TotalMilliseconds.ToString("#,##0.00000").PadLeft(10) + "ms");


        startTime2 = DateTime.Now;
        for (int x = 0; x < classes.Count; x++)
        {
            int age = classes[x].Age;
        }
        endTime2 = DateTime.Now;
        TimeSpan span2 = endTime2 - startTime2;
        Console.WriteLine("ForLoop Time:  " + span2.TotalMilliseconds.ToString("#,##0.00000").PadLeft(10) + "ms");
    }

    class MyClass
    {
        public int Age;

        public MyClass(int a)
        {
            Age = a;
        }
    }

Results:

Trial 1
ForEach Time:    15.60000ms
ForLoop Time:     0.00000ms

Trial 2
ForEach Time:     0.00000ms
ForLoop Time:    15.60000ms

Trial 3
ForEach Time:    15.60000ms
ForLoop Time:     0.00000ms

Trial 4
ForEach Time:    15.60000ms
ForLoop Time:     0.00000ms

Trial 5
ForEach Time:    15.60010ms
ForLoop Time:     0.00000ms

Trial 6
ForEach Time:     0.00000ms
ForLoop Time:     0.00000ms

Trial 7
ForEach Time:    15.60000ms
ForLoop Time:     0.00000ms

Trial 8
ForEach Time:    15.60010ms
ForLoop Time:     0.00000ms

Trial 9
ForEach Time:     0.00000ms
ForLoop Time:    15.60010ms

Trial 10
ForEach Time:     0.00000ms
ForLoop Time:     0.00000ms

score 2 · Accepted Answer · answered Nov 08 '16 at 01:58

2

You should be using System.Diagnostic.StopWatch class instead of using DateTime.Now for proper time elapsed comparison

answered Nov 08 '16 at 01:58

Rahul

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That fixed it. Do you mind giving insight on why Datetime.Now acts that way? – TchPowDog Nov 08 '16 at 02:06
1

@user2777664, Yeah sure. Check this post http://stackoverflow.com/questions/2923283/stopwatch-vs-using-system-datetime-now-for-timing-events (OR) MSDN documentation. In nutshell, `stopwatch` gives higher precision than `DateTime.Now` – Rahul Nov 08 '16 at 02:11
Thanks! I appreciate it. – TchPowDog Nov 08 '16 at 02:19

Foreach vs For loop weird results

1 Answers1