for vs foreach

foreach.cs

using System;
using System.Diagnostics;

namespace ConsoleApp11
{
    class Program
    {
        static long count_for(int[] a1, int[] a2)
        {
            long count = 0;
            for (int i = 0; i < a1.Length; i++)
            {
                for (int j = 0; j < a2.Length; j++)
                {
                    if (a1[i] + a2[j] % 3 == 0)
                    {
                        count++;
                    }                          
                }
            }
            return count;
        }

        static long count_foreach(int[] a1, int[] a2)
        {
            long count = 0;
            foreach (int a in a1) 
            {
                foreach (int b in a2) 
                {
                    if (a + b % 3 == 0)
                    {
                        count++;
                    }
                }
            }
            return count;
        }

        static void Main(string[] args)
        {
            int[] a1 = new int[20000];
            int[] b1 = new int[20000];
            for (int i = 0; i < a1.Length; i++)
            {
                a1[i] = i;
                b1[i] = i / 2;
            }

            var s = new Stopwatch();
            s.Start();
            long r_for = count_for(a1, b1);
            s.Stop();
            Console.WriteLine("for loop took:" + s.ElapsedMilliseconds + " answer:" + r_for);

            s.Restart();            
            long r_foreach = count_foreach(a1, b1);
            s.Stop();
            Console.WriteLine("foreach Took:" + s.ElapsedMilliseconds + " answer:" + r_foreach);

            s.Restart();
            r_for = count_for(a1, b1);
            s.Stop();
            Console.WriteLine("for loop Took:" + s.ElapsedMilliseconds + " answer:" + r_for);

            s.Restart();
            r_foreach = count_foreach(a1, b1);
            s.Stop();
            Console.WriteLine("foreach Took:" + s.ElapsedMilliseconds + " answer:" + r_foreach);
            Console.ReadLine();
        }
    }
}