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c# - 使用多态对象创建时出现继承错误

问题描述

我有一个 FootWearItem 类和一个 CartItem 类，它们都继承自 ProductItem 类。我还有一个 ShoppingCart 类，其中包含一个包含 CartItems 的列表。

我正在 Main(); 中测试这些类；我正在尝试将 FootWearItems 添加到 ShoppingCart，尽管我似乎收到一条错误消息，指出无法将 ProductItem 转换为 CartItem。

谁能告诉我这是为什么？

public abstract class ProductItem
    {
        public string ProductCode { get; set; }
        public string Description { get; set; }
        public double Price { get; set; }

        public ProductItem(string productCode, string description, double price)
        {
            ProductCode = productCode;
            Description = description;
            Price = price;
        }

        public override string ToString()
        {
            return "Product Code: " + ProductCode + "\nDescription: " + Description + "\nPrice: " + Price;
        }
    }


    public enum Gender
    {
        Men, Women
    }

    public class FootWearItem : ProductItem
    {
        public double Size { get; set; }
        public Gender Gender { get; set; }

        public FootWearItem(string productCode, string description, double price, double size, Gender gender) 
            : base (productCode, description, price)
        {
            Size = size;
            Gender = gender;
        }


        public override string ToString()
        {
            return base.ToString() + "\nSize: " + Size + "\nGender" + Gender;
        }
    }

    public class CartItem : ProductItem
    {
        public int NumberItems { get; set; }
        public CartItem(string productCode, string description, double price, int numberItems) 
            : base(productCode, description, price)
        {
            NumberItems = numberItems;
        }
    }

    public class ShoppingCart
    {
        List<CartItem> cart;
        public int MyProperty { get; set; }

        private double total;


        public ShoppingCart()
        {
            cart = new List<CartItem>();
        }

        public void AddItem(CartItem item)
        {
            if(!cart.Contains(item))
            {
                cart.Add(item);
            }
            else
            {
                throw new ArgumentException("Item is already in cart");
            }
        }
        public double TotalPrice()
        {
            foreach(CartItem i in cart)
            {
                total += i.Price;
            }

            return total;
        }
    }

    public class TestClass
    {
        public static void Main()
        {
            ProductItem i1 = new FootWearItem("A122", "black nike shoe", 50.99, 10.5, Gender.Men);
            ProductItem i2 = new FootWearItem("A122", "black nike shoe", 70.99, 9.5, Gender.Men);
            ProductItem i3 = new FootWearItem("A122", "black nike shoe", 60.99, 6, Gender.Women);

            ShoppingCart cart1 = new ShoppingCart();
            cart1.AddItem(i1);
            cart1.AddItem(i2);
            cart1.AddItem(i3);
        }
    }

干杯

标签： c#asp.net

解决方案

您的 CartItem 应该包含 ProductItem 作为属性——就目前而言，ProductItem 不是从 CartItem 派生的，因此不能强制转换。

public class CartItem
{
    public int NumberItems { get; set; }
    public ProductItem Product { get; set; }

    public CartItem(ProductItem product, int numberItems)
    {
        Product = product;
        NumberItems = numberItems;
    }
}

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