using System;

using System.Globalization;

using System.Linq;

​

public class Program

{

    /// <see cref="https://en.wikipedia.org/wiki/Metric_prefix#List_of_SI_prefixes" />

        public static void Main()

        {

            try

            {

                    var d0 = HumanizeBigNumber(1234567890);

                    Console.WriteLine(d0.PadRight(10) + DehumanizeBigNumber(d0));

                for (int i = 0; i < 27; i++)

                {

                    var d = HumanizeBigNumber(Math.Pow(10, i));

                    Console.WriteLine(d.PadRight(10) + DehumanizeBigNumber(d));

                }

            }

            catch (Exception ex)

            {

                Console.WriteLine(ex.Message);

            }

​

        }

​

        private static string HumanizeBigNumber(double bigNumber)

        {

            const string symbols = "kMGTPEZY";

            if (bigNumber > Math.Pow(10, 26))

                throw new ArgumentOutOfRangeException("bigNumber");

            var numbers = bigNumber

                .ToString("0.###E+0", CultureInfo.InvariantCulture) // scientific notation

                .Split(new[] { 'E', '+' }, StringSplitOptions.RemoveEmptyEntries) // split {significand}, {exponent}

                .Select(Double.Parse).ToArray();

​

            var number = numbers[0] * Math.Pow(10, numbers[1] % 3); // significand > 0 && < 1000

            var i = (numbers[1] / 3) - 1; // index to select in chars

            var letter = i >= 0 ? symbols[(int)i].ToString() : String.Empty; // get valid letter

            return number + letter;

        }

​

        private static double DehumanizeBigNumber(string humanizeBigNumber)

        {

            const string symbols = "kMGTPEZY";

            var letter = Char.IsLetter(humanizeBigNumber.Last()) ? humanizeBigNumber.Last().ToString() : String.Empty;

​

            return String.IsNullOrEmpty(letter)

                ? Double.Parse(humanizeBigNumber)

                : (Double.Parse(humanizeBigNumber.Remove(humanizeBigNumber.Length - 1))

                  * Math.Pow(10, 3 + symbols.IndexOf(letter, StringComparison.Ordinal) * 3));

        }

}