using System;

public class Program

{

    public static void Main()

    {

        Sales sales = new Sales(5);

        Order order = new Order();

        sales.ThersholdReached += order.Order_ThersholdReached;

        Item item = new Item {

            Id =1,

            Name ="Apple",

            Price = 100,

            StockInHand = 100

        };

        sales.SaveBill(item, 96);

    }

}

​

public class Item

{

    public int Id {get; set;}

    public string Name {get; set;}

    public double Price {get; set;}

    public long StockInHand {get; set;}

}

​

public class Sales

{

    private long thersholdLimit;

    public Sales (long thershold)

    {

        thersholdLimit = thershold;

    }

    public void SaveBill(Item item, long qty)

    {

        Console.WriteLine("The sales bill was success!");

        if((item.StockInHand - qty) < thersholdLimit)

        {

            OnThersholdReached(EventArgs.Empty);

        }

    }

    protected virtual void OnThersholdReached(EventArgs e)

    {

        EventHandler handler = ThersholdReached;

        if(handler != null)

        {

            handler(this, e);

        }

    }

    public event EventHandler ThersholdReached;

}

​

public class Order

{

    public void Order_ThersholdReached(object sender, EventArgs e)

    {

        Console.WriteLine("The stock was reached and invoked the new order!");

    }

}