public ExchangeRate(decimal rate, ExchangeSymbol exchangeSymbol, DateTime date, CultureInfo cultureInfo)
 {
     Rate         = rate;
     Date         = date;
     _cultureInfo = cultureInfo;
     Symbol       = exchangeSymbol ?? throw new ArgumentNullException(nameof(exchangeSymbol));
 }
 public ExchangeRate(decimal rate, string fromSymbol, string toSymbol, DateTime date, CultureInfo cultureInfo)
 {
     Rate         = rate;
     Date         = date;
     _cultureInfo = cultureInfo;
     Symbol       = new ExchangeSymbol(fromSymbol, toSymbol);
 }