WithIndex

Program.cs

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

namespace PlayWithIndex
{
    class Program
    {
        static void Main()
        {
            var list = new List<string> { "a", "b", "c", "d" };

            Console.WriteLine("foreach loop as item");
            foreach (var item in list.WithIndex())
                Console.WriteLine($"{item.Index} {item.Value}");

            Console.WriteLine("foreach loop with tuple values");
            foreach (var (value, index) in list.WithIndex())
                Console.WriteLine($"{index} {value}");

            Console.WriteLine("foreach method as item");
            list.WithIndex().ForEach(item => Console.WriteLine($"{item.Index} {item.Value}"));
        }
    }

    public static class LinqExtensions
    {
        public static IEnumerable<(T Value, int Index)> WithIndex<T>(this IEnumerable<T> source)
            => source.Select((item, index) => (item, index));

        public static void ForEach<T>(this IEnumerable<T> thisValue, Action<T> action)
        {
            foreach (var value in thisValue)
                action?.Invoke(value);
        }
    }
}