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);
}