Payment Example

payment.cs

using System;
using System.Collections.Generic;

namespace ClassLibrary3
{
    public class Program
    {
        public static void Main(string[] args)
        {
            var service = new PaymentService(
                new CompositeDiscountableFactory(
                    // new EventDiscountFactory(),
                    new NaverDiscountableFactory(),
                    new DanawaDiscountableFactory(),
                    new FancafeDiscountableFactory()));

            service.Pay(100, "Naver");
            service.Pay(100, "Danawa");
            service.Pay(100, "Fancafe");
        }
    }

    public class PaymentService
    {
        public PaymentService(IDiscountableFactory factory)
        {
            Factory = factory;
        }

        public IDiscountableFactory Factory { get; }

        public void Pay(decimal amount, string discountName)
        {
            decimal payment = amount - Factory.Create(discountName).GetDiscountAmount(amount);
            Console.WriteLine($"Paied {payment}.");
        }
    }

    public interface IDiscountable
    {
        decimal GetDiscountAmount(decimal originalAmout);
    }

    public class NullDiscountPolicy : IDiscountable
    {
        private static NullDiscountPolicy instance = new NullDiscountPolicy();

        private NullDiscountPolicy()
        {
        }

        public static NullDiscountPolicy Instance
        {
            get { return NullDiscountPolicy.instance; }
        }

        public decimal GetDiscountAmount(decimal originalAmout)
        {
            throw new InvalidOperationException(
                "This respresents the null-discount policy.");
        }
    }

    public class RateDiscountPolicy : IDiscountable
    {
        public RateDiscountPolicy(decimal rate)
        {
            Rate = rate;
        }

        public decimal Rate { get; }

        public decimal GetDiscountAmount(decimal originalAmout)
        {
            return originalAmout * Rate;
        }
    }

    public class MaxDiscountPolicy : IDiscountable
    {
        public MaxDiscountPolicy(decimal maxAmount)
        {
            MaxAmount = maxAmount;
        }

        public decimal MaxAmount { get; }

        public decimal GetDiscountAmount(decimal originalAmout)
        {
            return originalAmout < MaxAmount
                ? originalAmout
                : MaxAmount;
        }
    }

    public interface IDiscountableFactory
    {
        IDiscountable Create(string discountName);
    }

    public class CompositeDiscountableFactory : IDiscountableFactory
    {
        public CompositeDiscountableFactory(params IDiscountableFactory[] factories)
        {
            Factories = factories;
        }

        IEnumerable<IDiscountableFactory> Factories { get; }

        public IDiscountable Create(string discountName)
        {
            foreach (IDiscountableFactory factory in Factories)
            {
                IDiscountable policy = factory.Create(discountName);
                if (policy != NullDiscountPolicy.Instance)
                    return policy;
            }

            return NullDiscountPolicy.Instance;
        }
    }

    public class NaverDiscountableFactory : IDiscountableFactory
    {
        public IDiscountable Create(string discountName)
        {
            return discountName == "Naver"
                    ? (IDiscountable)new RateDiscountPolicy(0.1m)
                    : NullDiscountPolicy.Instance;
        }
    }

    public class DanawaDiscountableFactory : IDiscountableFactory
    {
        public IDiscountable Create(string discountName)
        {
            return discountName == "Danawa"
                    ? (IDiscountable)new RateDiscountPolicy(0.15m)
                    : NullDiscountPolicy.Instance;
        }
    }

    public class FancafeDiscountableFactory : IDiscountableFactory
    {
        public IDiscountable Create(string discountName)
        {
            return discountName == "Fancafe"
                   ? (IDiscountable)new MaxDiscountPolicy(50m)
                   : NullDiscountPolicy.Instance;
        }
    }

    public class EventDiscountFactory : IDiscountableFactory
    {
        public IDiscountable Create(string discountName)
        {
            //DB에서 dicountName에 해당하는 할인율 적용
            decimal rate;
            if (TryGetDicountRate(discountName, out rate))
            {
                return new RateDiscountPolicy(rate);
            }

            return NullDiscountPolicy.Instance;
        }

        private bool TryGetDicountRate(string discountName, out decimal rate)
        {
            rate = 0.2m; // DB에서 할인율 가져오기
            return true;
        }
    }
}

코드 보고 잘 배우고 갑니다.
어떤 고민을 하셨는지 간접적으로 느껴지는 집니다.

코드 잘 보았습니다. 그런데 질문이 있습니다
코드에서 새로운 DiscountPolicy를 추가하려면 결국에는 Main 메서드가 있는 Program 클래스를 수정해야하는데, 그러면 결국 OCP가 또 위반이 되는게 아닌가 생각합니다