As is often the case with newish people, I don't think you've actually asked the right question. What's your real requirement here? From reading the thread, it appears to be that you want a surcharge, either a percentage or a fixed amount, to be applied to a bill. What's the interface to that?
interface ISurcharge {
double GetAmount(Order order);
string Name { get; }
}
... or, if it's just on the amount, you can pass a number instead of an order. My guess, though, is different tax rates may apply to different items so you will eventually need to pass the order. Note the important thing here: the value of the surcharge depends (in general) on the existing order so it needs to be part of the interface. The Name property is for what gets displayed on the bill; you may want a few other little things like that, but this is the core functionality. Now you can create trivial implementation classes to solve the problem!
class FixedSurcharge : ISurcharge {
private double amount;
public string Name { get; private set; }
public FixedSurcharge(string name, double amount) { Name = name; this.amount = amount; }
public double GetAmount(Order order) { return amount; }
}
class PercentageSurcharge : ISurcharge {
private double percentage;
public string Name { get; private set; }
public PercentageSurcharge (string name, double percentage) { Name = name; this.percentage = percentage; }
public double GetAmount(Order order) { return percentage * 0.01 * order.TotalValue; }
}
... called like:
void placeOrder(Order order){
foreach(OrderItem item in order.Items) { addBillItem(item.Name, item.Value); }
addSurcharge(new PercentageSurcharge("VAT", 20), order);
addSurcharge(new FixedSurcharge("Shipping", 2.50), order);
}
void addSurcharge(ISurcharge surcharge, Order order){
addBillItem(surcharge.Name, surcharge.GetAmount(order));
}
(Actually, on reflection it may be cleaner to make fake order items, with a Value property, by passing the existing Order to a constructor and stashing it in the Surcharge instance. That's a matter of style, I think.)
