c#中设计一个超市打折系统

在C#中设计一个简单的超市打折系统，我们可以考虑以下几个步骤：

1. **实体类**:
- **商品类** (Product): 包含基本属性如名称、原价和ID，以及折扣信息。

   public class Product
   {
       public int Id { get; set; }
       public string Name { get; set; }
       public decimal OriginalPrice { get; set; }
       public double DiscountRate { get; set; } // 折扣率，通常范围0-1
   }

2. **优惠策略类** (PromotionStrategy): 定义一种或多种打折规则，比如满额减、特定日期优惠等。

   public abstract class PromotionStrategy
   {
       public abstract decimal CalculateDiscount(Product product);
   }

   // 具体的促销策略例如满额减
   public class FixedAmountDiscount : PromotionStrategy
   {
       private decimal _discountThreshold;

       public FixedAmountDiscount(decimal discountThreshold)
       {
           _discountThreshold = discountThreshold;
       }

       public override decimal CalculateDiscount(Product product)
       {
           if (product.OriginalPrice >= _discountThreshold)
               return product.OriginalPrice * DiscountRate;
           else
               return 0;
       }
   }

3. **购物车类** (ShoppingCart) 或 **订单类** (Order): 存储商品列表，并能应用优惠策略计算总价。

   public class ShoppingCart
   {
       private List<Product> products = new List<Product>();
       private PromotionStrategy promotionStrategy;

       public void AddProduct(Product product)
       {
           products.Add(product);
       }

       public void ApplyPromotion(PromotionStrategy strategy)
       {
           foreach (var product in products)
               product.Price = product.CalculateFinalPrice(strategy);
       }

       private decimal CalculateFinalPrice(Product product)
       {
           return product.OriginalPrice * (1 - promotionStrategy.CalculateDiscount(product));
       }
   }

4. **主程序** (Main):
用户添加商品到购物车，然后选择并应用合适的优惠策略。

   static void Main(string[] args)
   {
       var cart = new ShoppingCart();
       cart.AddProduct(new Product("牛奶", 10, 0.9)); // 添加商品
       cart.AddProduct(new Product("面包", 5, 0.8));

       var promotion = new FixedAmountDiscount(15); // 创建满20元减5元的策略
       cart.ApplyPromotion(promotion);

       Console.WriteLine($"购物车总额: {cart.TotalPrice}");
   }