using System;

using System.Collections.Generic;

using System.Collections.ObjectModel;

using System.Linq;

​

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 readonly List<OrderItem> _orderItems = new List<OrderItem>();

​

    public IReadOnlyList<OrderItem> OrderItems

    {

        get

        {

            return new ReadOnlyCollection<OrderItem>(_orderItems);

        }

    }

​

    public void AddRange(IEnumerable<OrderItem> orderItems)

    {

        _orderItems.AddRange(orderItems);

    }

}

​

public class Program

{

    public static void Main()

    {

        var order = new Order();

​

        order.AddRange(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)

                           });

​

        Display("Order #1", order);

​

        var item = order.OrderItems.SingleOrDefault(x => x.PartNumber == 111033401);

​

        if (item != null)

        {

            Console.WriteLine("Order #1 has #111033401 item - price is {0:###,###.00}$.\n", item.UnitPrice * item.Quantity);

        }

        else

        {

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

        }

​

        order.AddRange(new OrderItem[]

                           {

                               new OrderItem(111033401, "Nut", 10, .5),

                               new OrderItem(127700026, "Crank", 27, 5.98)

                           });

​

        Display("Order #2", order);

​

        item = order.OrderItems.SingleOrDefault(x => x.PartNumber == 127700026);

​

        if (item != null)

        {

            Console.WriteLine("Order #2 has #127700026 item - price is {0:###,###.00}$.\n", item.UnitPrice * item.Quantity);

        }

        else

        {

            Console.WriteLine("Order #2 doesn't have #127700026 item.\n");

        }

    }

    private static void Display(string title, Order order)

    {

        Console.WriteLine(title);

        foreach (OrderItem item in order.OrderItems)

        {

            Console.WriteLine(item);

        }

        Console.WriteLine();

    }

}