OnlineSupermarket.Cart.cs

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using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;

namespace OnlineSupermarket
{
    public sealed partial class OnlineSupermarket
    {
        /// <summary>
        /// 表示一辆购物车。
        /// </summary>
        public sealed class Cart : ICollection<GoodsInfo>
        {
            private readonly List<GoodsInfo> goods;

            public Cart()
            {
                this.goods = new List<GoodsInfo>();
            }

            public int Count => this.goods.Count;
            bool ICollection<GoodsInfo>.IsReadOnly => false;

            public void Add(GoodsInfo goods)
            {
                if (goods == null)
                    throw new ArgumentNullException(nameof(goods));
                this.goods.Add(goods);
            }

            public bool Remove(GoodsInfo goods)
            {
                if (goods == null)
                    throw new ArgumentNullException(nameof(goods));
                return this.goods.Remove(goods);
            }

            public void Clear()
            {
                this.goods.Clear();
            }

            public bool Contains(GoodsInfo? goods)
            {
                if (goods == null)
                    return false;
                return this.goods.Contains(goods);
            }

            /// <summary>
            /// 结账取得收据。
            /// 若要正确结账，购物车里的所有物品都在线上超市中存在，否则返回 <c>null</c> 。
            /// </summary>
            /// <param name="supermarkets">用以判断物品是否存在的线上超市。</param>
            /// <returns>收据。</returns>
            public Receipt? CheckOutIn(params OnlineSupermarket[] supermarkets)
            {
                if (supermarkets == null)
                    return null;