using System;

using System.Collections.Generic;

using System.Collections.ObjectModel;

​

public class OrderItem

{

    public readonly int PartNumber;

    public readonly string Description;

    public readonly double UnitPrice;

    private int _quantity = 0;

    public OrderItem(int partNumber, string description, int quantity, double unitPrice)

    {

        this.PartNumber = partNumber;

        this.Description = description;

        this.Quantity = quantity;

        this.UnitPrice = unitPrice;

    } 

    public int Quantity    

    {

        get

        {

            return _quantity;

        }

        set

        {

            if (value < 0)

            {

                throw new ArgumentException("Quantity cannot be negative.");

            }

​

            _quantity = value;

        }

    }

    public override string ToString()

    {

        return String.Format("{0,9} {1,6} {2,-12} at {3,8:#,###.00} = {4,10:###,###.00}", 

            PartNumber, _quantity, Description, UnitPrice, UnitPrice * _quantity);

    }

}

​

public class Order

{

    private OrderItem[] _orderItems;

​

    public OrderItem[] OrderItems

    {

        get

        {

            return _orderItems;

        }

        set

        {

            if (value == null)

            {

                throw new ArgumentNullException("value");

            }

            this._orderItems = value;

        }

    }

    public Order(OrderItem[] orderItems)

    {

        this.OrderItems = orderItems;

    }

}

​

​

public class Program

{

    public static void Main()

    {

        var orderItems = new OrderItem[]

        {

            new OrderItem(110072674, "Widget", 400, 45.17),

            new OrderItem(110072675, "Sprocket", 27, 5.3),

            new OrderItem(101030411, "Motor", 10, 237.5),

            new OrderItem(110072684, "Gear", 175, 5.17)

        };

        var order = new Order(orderItems);

        Display("Order #1", order);

        if (!Array.Exists(order.OrderItems, item => item.PartNumber == 111033401))

        {

            Console.WriteLine("Order #1 doesn't have #111033401 item.\n");

        }

​

        var newOrderItems = new OrderItem[orderItems.Length + 2];

        Array.Copy(orderItems, 0, newOrderItems, 0, orderItems.Length);

​

        newOrderItems[orderItems.Length] = new OrderItem(111033401, "Nut", 10, .5);

        newOrderItems[orderItems.Length + 1] = new OrderItem(127700026, "Crank", 27, 5.98);

​

        order.OrderItems = newOrderItems;

        Display("Order #2", order);

​

        if (Array.Exists(order.OrderItems, item => item.PartNumber == 127700026))

        {

           Console.WriteLine("Order #2 has #127700026 item.");

        }

    }

    private static void Display(string title, Order order)

    {

        Console.WriteLine(title);

        foreach (OrderItem item in order.OrderItems)

        {

            Console.WriteLine(item);

        }

        Console.WriteLine();

    }

}