C# 4 "Local Function" example with tuples in a tail call

Program.cs

using System;

namespace TupleExample
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine($"1! = {Factorial(1)}");
            Console.WriteLine($"5! = {Factorial(5)}");
            Console.WriteLine($"20! = {Factorial(20)}");

            Console.ReadKey();
        }

        private static long Factorial(int x) => FactorialAux(new Tuple<int, int>(1, x));

        private static long FactorialAux(Tuple<int, int> t)
        {
            if (t.Item2 <= 1) return t.Item1;

            var r = new Tuple<int, int>(t.Item1*t.Item2, t.Item2 - 1);
            return FactorialAux(r);
        }
    }
}

Pairs with https://gist.github.com/MikeMKH/f5e2bf37b384308d25d54793d9e4f752