MoneyTest.cs

using System;

namespace MoneyTest
{

    public static class Program
    {
        static void Main(string[] args)
        {
            var twentyEuro = new Money(20.0, Currency.EUR);
            var twentyDollar = new Money(20.0, Currency.USD);

            var handler = new ConversionHandler(new ExchangeRateService());
            var command = new ConversionCommand { Money = twentyDollar, ToCurrency = Currency.EUR };
            var twentyDollarInEuro = handler.Handle(command);

            Console.WriteLine("20 USD in euro: " + twentyDollarInEuro);
            var sum = twentyEuro + twentyDollarInEuro;
            Console.WriteLine("Result 20 EUR + 20 USD: " + sum);

            Console.ReadLine();
        }
    }

    public enum Currency
    {
        USD,
        EUR
    }

    public class ConversionCommand
    {
        public Money Money { get; set; }
        public Currency ToCurrency { get; set; }
    }

    public sealed class ConversionHandler
    {
        private readonly IDetermineExchangeRate _exchangeRateService;

        public ConversionHandler(IDetermineExchangeRate exchangeRateService)
        {
            _exchangeRateService = exchangeRateService;
        }

        public Money Handle(ConversionCommand command)
        {
            var exchangeRate = _exchangeRateService.DetermineExchangeRate(command.Money.Currency, command.ToCurrency);
            return new Money(exchangeRate * command.Money.Amount, command.ToCurrency);
        }
    }

    public interface IDetermineExchangeRate
    {
        double DetermineExchangeRate(Currency fromCurrency, Currency toCurrency);
    }

    public class ExchangeRateService : IDetermineExchangeRate
    {
        public double DetermineExchangeRate(Currency fromCurrency, Currency toCurrency)
        {
            if (fromCurrency == toCurrency)
                return 1;

            if (fromCurrency == Currency.EUR && toCurrency == Currency.USD)
                return 1.37;
            if (fromCurrency == Currency.USD && toCurrency == Currency.EUR)
                return 0.72;

            throw new ArgumentException(string.Format("No exchange rate available for {0} to {1}", fromCurrency, toCurrency));
        }

    }

    public sealed class Money
    {
        public double Amount { get; private set; }
        public Currency Currency { get; private set; }

        public Money(double amount, Currency currency)
        {
            Amount = amount;
            Currency = currency;
        }

        public static Money operator +(Money money1, Money money2)
        {
            CheckThatCurrenciesAreTheSame(money1, money2);

            return new Money(money1.Amount + money2.Amount, money1.Currency);
        }

        public static Money operator -(Money money1, Money money2)
        {
            CheckThatCurrenciesAreTheSame(money1, money2);

            return new Money(money1.Amount - money2.Amount, money1.Currency);
        }

        private static void CheckThatCurrenciesAreTheSame(Money money1, Money money2)
        {
            if (money1.Currency != money2.Currency)
            {
                throw new ArgumentException(
                    string.Format("Can't add {0} to {1}", money2.Currency, money1.Currency));
            }
        }

        private bool Equals(Money other)
        {
            return Amount.Equals(other.Amount) && Currency == other.Currency;
        }

        public override bool Equals(object obj)
        {
            if (ReferenceEquals(null, obj)) return false;
            if (ReferenceEquals(this, obj)) return true;
            if (obj.GetType() != GetType()) return false;
            return Equals((Money)obj);
        }

        public override int GetHashCode()
        {
            unchecked
            {
                return (Amount.GetHashCode() * 397) ^ (int)Currency;
            }
        }


        public override string ToString()
        {
            return string.Format("{0} {1}", Amount, Currency);
        }
    }
}