IndexedEnumerable.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace ConsoleApp
{
    public static class Program
    {
        static void Main(string[] args)
        {
            var items = new List<string> { "kirk", "spock", "bones", "scotty" };

            foreach ((int index, string item) in items.Indexed())
            {
                Console.WriteLine($"item[{index}] is {item}");
            }
        }
    }

    public static class Sequence
    {
        public static IEnumerable<(int, T)> Indexed<T>(this IEnumerable<T> sequence) =>
            sequence.Select((value, index) => (index, value));
    }
}