Formatting numbers alternative

number-formatter.cs

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

namespace NumberFormatter
{
    class Program
    {
        private static Formatter formatter = new Formatter();
        static void Main(string[] args)
        {
            var numbers = new double[] { 10, 1234, 15000, 7500000, 19291921, 129312932193219, 1239812379128379281, 12389721983721983217111d };
            var numbersFormatted = numbers.Select(formatter.Format).ToArray();
            Console.WriteLine(String.Join("\n", numbersFormatted));
        }

    }

    public class Formatter
    {
        private Dictionary<double, string> names = new Dictionary<double, string>
        {
            { 1000, "k" } ,
            { 1000000, "million" },
            { 1000000000, "billion" },
            { 1000000000000, "trillion" },
            { 1000000000000000, "quadrillion" },
            { 1000000000000000000, "quintilion" },
            { 1000000000000000000000d, "sextillion" },
        };
        public string Format(double d)
        {
            var unit = names.Reverse().Where(e => d >= e.Key).FirstOrDefault();
            if (string.IsNullOrWhiteSpace(unit.Value)) return d.ToString();
            return (d / unit.Key).ToString("f3") + " " + unit.Value;
        }
    }
}