A purchase order has an order ID, a customer ID, total price and line items. Line items can be a product or membership type.

Business rules:

1. If the purchase order contains a membership, it has to be activated in the customer account immediately.
2. If the purchase order contains Comprehensive First Aid Training video then Basic First Aid training video is added to the purchase order

public abstract class Product
{
}
 public class BookClubMembership : Product
{
    public bool IsActivated { get; set; }        
}

Using the strategy design pattern, i define the first rule as follows

public class BookClubMembershipRule : IPurchaseOrderRule
{
    readonly ICustomerActivationService _customerActivationService;

    public BookClubMembershipRule(ICustomerActivationService customerActivationService)
    {
        _customerActivationService = customerActivationService;
    }
    public void Execute(PurchaseOrder order)
    {
        foreach(var product in order.Items) {
            if (product is BookClubMembership)
            {
                _customerActivationService.ActivateBookMembership(order.CustomerId, product as BookClubMembership);
                break;
            }
        }
        
    }
}

public class PurchaseOrderProcessor
{
    public IList<IPurchaseOrderRule> Rules { get; }

    public PurchaseOrderProcessor(List<IPurchaseOrderRule> rules)
    {
        Rules = rules;
    }

    public void ProcessOrder(PurchaseOrder order)
    {
        foreach (var rule in Rules)
            rule.Execute(order);
    }

I dont feel comfortable using C# is and as above. Is there a better way for me to program these rules. I was thinking of Visitor pattern but given the 2nd rule. I will need to operate on the order and the item as well.

A line item is what i call an item in an order. For example, an order can look like this.

Order No: 3344656
 Total: 48.50 
 Customer Id: 4567890 
 Line items:
  • Video "Comprehensive First Aid Training" 
  • Book "The Girl on the train" 
  • Book Club Membership 

Added code for the second rule:

public class ComprehensiveVideoRule : IPurchaseOrderRule
{
    public void Execute(PurchaseOrder order)
    {
        bool addNewVideo = false;
        foreach(var item in order.Items)
        {
            if(item is Video)
            {
                var checkItem = item as Video;
                if(checkItem.Title.Contains("Comprehensive"))
                    addNewVideo = true;
            }
        }
        if(addNewVideo) order.Items.Add(new Video("Basic First Aid"));
    }
}