如何在.NET核心API中设计嵌套类以获得特定的模式？

public class Order
{
    public int OrderId { get; set; }
    public string CustomerName { get; set; }
    public List<OrderItem> Items { get; set; } = new List<OrderItem>();

    // 静态嵌套类，用于创建订单
    public static class OrderFactory
    {
        public static Order CreateOrder(int orderId, string customerName)
        {
            return new Order
            {
                OrderId = orderId,
                CustomerName = customerName
            };
        }
    }

    // 非静态嵌套类，表示订单项
    public class OrderItem
    {
        public int ItemId { get; set; }
        public string ProductName { get; set; }
        public decimal Price { get; set; }
        public int Quantity { get; set; }

        public decimal TotalPrice => Price * Quantity;
    }
}

// 使用示例
public class Program
{
    public static void Main()
    {
        var order = Order.OrderFactory.CreateOrder(1, "John Doe");
        order.Items.Add(new Order.OrderItem
        {
            ItemId = 101,
            ProductName = "Laptop",
            Price = 999.99m,
            Quantity = 1
        });

        order.Items.Add(new Order.OrderItem
        {
            ItemId = 102,
            ProductName = "Smartphone",
            Price = 499.99m,
            Quantity = 2
        });

        Console.WriteLine($"Order ID: {order.OrderId}");
        Console.WriteLine($"Customer Name: {order.CustomerName}");
        foreach (var item in order.Items)
        {
            Console.WriteLine($"Product: {item.ProductName}, Price: {item.Price}, Quantity: {item.Quantity}, Total: {item.TotalPrice}");
        }
    }
}