• It works by separating out the rules from the rules processing logic.
  • This makes it easy to add new rules without changing the rest of the system.
• With the Rules Pattern there is an Evaluator class that loops through a collection of rules and executes them.
  • It evaluates the result and decides what action to take.
  • In the simplest case, it just executes all the rule.
    • It is also possible to add some selection logic to each rule that allows the Evaluator class to decide whether or not to run the rule.
• Applying the refactoring to the problems, the Evaluator will have a collection of rules that calculate rules.

public class Customer
{
	public string Name { get; set; }
	public DateTime BirthDay { get; set; }
	public bool HasBeenLoyalForYears(int yearsAsCustomer, DateTime date) => true;
	public bool IsBirthday() => BirthDay.Date == DateTime.Now.Date;
}

public interface IDiscountRule
{
	decimal CalculateCustomerDiscount(Customer customer);
}

public class BirthdayDiscountRule : IDiscountRule
{
	public decimal CalculateCustomerDiscount(Customer customer)
	{
		return customer.IsBirthday() ? 0.10m : 0;
	}
}

public class LoyalCustomerRule : IDiscountRule
{
	private readonly int _yearsAsCustomer;
	private readonly decimal _discount;
	private readonly DateTime _date;

	public LoyalCustomerRule(int yearsAsCustomer, decimal discount, DateTime? date = null)
	{
		_yearsAsCustomer = yearsAsCustomer;
		_discount = discount;
		_date = date ?? DateTime.Now;
	}

	public decimal CalculateCustomerDiscount(Customer customer)
	{
		if (customer.HasBeenLoyalForYears(_yearsAsCustomer, _date))
		{
			var birthdayRule = new BirthdayDiscountRule();
			return _discount + birthdayRule.CalculateCustomerDiscount(customer);
		}
		return 0;
	}
}

public class RulesDiscountCalculator
{
	List<IDiscountRule> _rules = new List<IDiscountRule>();

	public RulesDiscountCalculator()
	{
		_rules.Add(new BirthdayDiscountRule());
		_rules.Add(new LoyalCustomerRule(1, 0.10m));
		_rules.Add(new LoyalCustomerRule(5, 0.12m));
		_rules.Add(new LoyalCustomerRule(10, 0.20m));
	}

	public decimal CalculateDiscountPercentage(Customer customer)
	{
		decimal discount = 0;
		foreach (var rule in _rules)
			discount = Math.Max(rule.CalculateCustomerDiscount(customer), discount);
		return discount;
	}
}