Question

I have a class Voucher:

public abstract class Voucher
    {
        public int Id { get; set; }
        public decimal Value { get; protected set; }
        public const string SuccessMessage = "Applied";
    }

and a subclass GiftVoucher

public class GiftVoucher : Voucher
    {
    }

and another subclass DiscountVoucher

public class DiscountVoucher : Voucher
    {
        public decimal Threshold { get; private set; }
        public string FailureMessage { get { return "Please spend £{0} to use this discount"; } }
    }

You can see that DiscountVoucher has a couple of specific properties Threshold and FailureMessage that respectively represent the amount of money you need to spend to get the discount and the failure message to display if the user has not spent that money.

My question is this. I have a collection of Voucher objects and what I don’t want to do in my code is something like this

if (voucher is DiscountVoucher)
{
   // cast voucher to a DiscountVoucher and then call the specific methods on it
} 

because this is not at all maintainable. At the same time I did not want to put those specific methods in the Voucher abstract class because they are not applicable to all types of Vouchers. Does anyone know how to design this functionality?

Answer 1

Well what you’ve got here is a version of the strategy pattern. I don’t think there’s any getting away from eventually having to decide if you have one type of voucher or another but you can limit the number of variations – voucher categories if you will – using interfaces.

For instance you might end up with five vouchers which implement interfaces called ‘StandardVoucher’ and three called ‘DiscountVoucher’ but instead of having to handle eight cases you now just have two.

The interfaces can cover a range of vouchers showing the available methods without worrying about the details of each vouchers implementation.