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 OrderItemCollection _orderItems;

​

    public OrderItemReadOnlyCollection OrderItems

    {

        get

        {

            return new OrderItemReadOnlyCollection(_orderItems);

        }

    }

    public Order()

    {

        _orderItems = new OrderItemCollection();

        _orderItems.OrderItemAdded += OrderItemAdded;

        _orderItems.OrderItemRemoved += OrderItemRemoved;

    }

    public Order(IEnumerable<OrderItem> orderItems):this()

    {

        AddRange(orderItems);   

    }

    public void AddOrderItem(OrderItem orderItem)

    {

        if (orderItem == null)

        {

            throw new ArgumentNullException(String.Format("{0} can't be null", orderItem)); 

        }

        _orderItems.AddItem(orderItem);

    }

    public void RemoveOrderItem(OrderItem orderItem)

    {

        if (orderItem == null)

        {

            throw new ArgumentNullException(String.Format("{0} can't be null", orderItem)); 

        }

        _orderItems.RemoveItem(orderItem);

    }

    public void AddRange(IEnumerable<OrderItem> orderItems)

    {

        if (orderItems == null)

        {

            throw new ArgumentNullException(String.Format("{0} can't be null", orderItems));    

        }   

        foreach (OrderItem item in orderItems)

        {

            _orderItems.AddItem(item);  

        }

    }

    private void OrderItemAdded(object sender, OrderItemCollectionEventArgs eventArgs)

    {

        Console.WriteLine(String.Format("Order item with number {0} was added", eventArgs.OrderItem.PartNumber));

    }

    private void OrderItemRemoved(object sender, OrderItemCollectionEventArgs eventArgs)

    {

        Console.WriteLine(String.Format("Order item with number {0} was removed", eventArgs.OrderItem.PartNumber));

    }

}

​

public class OrderItemCollection: Collection<OrderItem>

{

    public event EventHandler<OrderItemCollectionEventArgs> OrderItemAdded = delegate {};

    public event EventHandler<OrderItemCollectionEventArgs> OrderItemRemoved = delegate {};

    public void AddItem(OrderItem orderItem)

    {

        base.Add(orderItem);

    }

    public void RemoveItem(OrderItem orderItem)

    {

        base.Remove(orderItem);

    }

    protected virtual void OnAddingOrderItem(OrderItemCollectionEventArgs eventArgs)

    {

        this.OrderItemAdded.Invoke(this, eventArgs);

    }

    protected virtual void OnRemovingOrderItem(OrderItemCollectionEventArgs eventArgs)

    {

        this.OrderItemRemoved.Invoke(this, eventArgs);

    }

}

​

public class OrderItemReadOnlyCollection: ReadOnlyCollection<OrderItem>

{

    public OrderItemReadOnlyCollection(IList<OrderItem> orderItems) :base(orderItems)

    { }

}

​

public class OrderItemCollectionEventArgs: EventArgs

{

    private OrderItem _orderItem;

    public OrderItem OrderItem

    {

        get

        {

            return _orderItem;

        }

    }

    public OrderItemCollectionEventArgs(OrderItem orderItem)

    {

        this._orderItem = orderItem;

    }

}

​

​

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);

    }

    private static void Display(string title, Order order)

    {

        Console.WriteLine(title);

        foreach (OrderItem item in order.OrderItems)

        {

            Console.WriteLine(item);

        }

        Console.WriteLine();

    }

}