exceptions

class NumberFormatException : Exception
        {
            public NumberFormatException (string qty) : base(qty) { }
        }
        class StringFormatException : Exception
        {
            public StringFormatException (string name) : base (name) { }
        }
        class CurrencyFormatException : Exception
        {
            public CurrencyFormatException (string price) : base (price) { }
        }