class Delivery
    {
        public int Id { get; set; }
        public string CompanyName { get; set; }
        public string Number { get; set; }
        public Order Order { get; set; }
        public int OrderId { get; set; }
    }

    class DeliveryConfig : IEntityTypeConfiguration<Delivery>
    {
        public void Configure(EntityTypeBuilder<Delivery> builder)
        {
            builder.ToTable("delivery");
        }
    }

    class Order
    {
        public int Id { get; set; }
        public string Name { get; set; }
        public Delivery Delivery { get; set; }
    }

    class OrderConfig : IEntityTypeConfiguration<Order>
    {
        public void Configure(EntityTypeBuilder<Order> builder)
        {
            builder.ToTable("order");
            // 设置一对一映射，需要HasForeignKey指定OrderId为外键
            builder.HasOne<Delivery>(o => o.Delivery).WithOne(d => d.Order).HasForeignKey<Delivery>(d => d.OrderId);
        }
    }

        private static Task GetOrder(MyDbContext ctx)
        {
            // 不使用include()，拿不到对应的order，但是可以拿到orderId。
            Delivery d = ctx.Deliveries.FirstOrDefault();
            // 使用include()，可以拿到对应的delivery。
            //Order o = ctx.Orders.Include(o => o.Delivery).FirstOrDefault();

            Console.WriteLine(d.CompanyName);
            return Task.CompletedTask;
        }

        private static async Task SaveOrder(MyDbContext ctx)
        {
            Order o = new Order()
            {
                Name = "order1",
            };

            Delivery d = new Delivery()
            {
                CompanyName = "company01",
                Order = o,
            };

            ctx.Deliveries.Add(d);
            await ctx.SaveChangesAsync();
        }