public void ChangeExchangeRate(int rateIndex, ExchangeRate newExchangeRate)
 {
     _rates[rateIndex]._ExchangeRate = newExchangeRate._ExchangeRate;
 }
        public void AddExchangeRate(Currency startcurrency, Currency endcurrency, float rate)
        {
            ExchangeRate Rate = new ExchangeRate(startcurrency, endcurrency, rate);

            _rates.Add(Rate);
        }