public static class Helpers
    {
        public static IEnumerable<(T item, int index)> WithIndex<T>(this IEnumerable<T> source)
        {
            return source.Select((item, index) => (item, index));
        }
    }


    class Program
    {
        static void Main(string[] args)
        {
            Stopwatch sw = new Stopwatch();
            string testItem = "";
            int index = 0;
            var list = new List<string>();
            for (int j = 0; j < 10000000; j++)
            {
                list.Add("1");
            }

         
            sw.Start();
            int i = 0;
            foreach (var item in list)
            {
                testItem = item;
                index = i++;
            }
            sw.Stop();
            var res = sw.ElapsedMilliseconds;
            sw.Reset();


            sw.Start();
            foreach (var (item, j) in list.WithIndex())
            {
                testItem = item;
                index = i;
            }
            sw.Stop();
            var res2 = sw.ElapsedMilliseconds;
            sw.Reset();


            Console.WriteLine("res 1 " + res + ", res 2 " + res2);
            Console.ReadKey();
        }
    }