using System;

using System.Globalization;

using System.Linq;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("input | expect | actual");

        Test(45);

        Test(99);

        Test(100);

        Test(200);

        Test(400);

        Test(980);

        Test(1200);

        Test(9999);

    }

    public static void Test(long val, string currency = "GBP") {

        Console.WriteLine(val.ToString("#.00")

                          + " | " + val.ToMoneyUnit_ProposedFix(currency).ToString("0.00")

                          + " | " + val.ToMoneyUnit(currency).ToString("0.00")

                    );

    }

}

​

​

​

public static class CurrencyExtensions

{

    public static int ToSmallestUnit(this decimal amount, string currencyCode)

    {

        var culture = GetCulture(currencyCode);

        int precision = culture.NumberFormat.CurrencyDecimalDigits;

        return (int)(amount * (decimal)Math.Pow(10, precision));

    }

    public static decimal ToMoneyUnit(this long amount, string currencyCode)

    {

        var culture = GetCulture(currencyCode);

        int precision = culture.NumberFormat.CurrencyDecimalDigits;

        int devisor = precision * 10;

        return ((decimal)amount) / devisor;

    }

    public static decimal ToMoneyUnit_ProposedFix(this long amount, string currencyCode)

    {

        var culture = GetCulture(currencyCode);

        int precision = culture.NumberFormat.CurrencyDecimalDigits;

        int devisor = (int)Math.Pow(10,precision);

        return ((decimal)amount) / devisor;

    }

    public static CultureInfo GetCulture(string currencyCode)

        {

            string cultureCode = null;

            switch (currencyCode.ToUpper())

            {

                case "GBP":

                    cultureCode = "en-GB";

                    break;

                case "USD":

                    cultureCode = "en-US";

                    break;

                case "EUR":

                    cultureCode = "de-DE";

                    break;

                default:

                    CultureInfo match = CultureInfo.GetCultures(CultureTypes.SpecificCultures)

                        .FirstOrDefault(x => new RegionInfo(x.LCID).ISOCurrencySymbol == currencyCode);

                    if (match != null)

                        return match;

                    throw new NotImplementedException("The currency code '"+currencyCode+"' is not currently supported");

            }

            return new CultureInfo(cultureCode);

        }

}