// 1 .-------------------------------------------------

    class UpdateEventArgs : EventArgs
    {
        public UpdateEventArgs(Int32 id, Int32 count)
        {
            m_id = id;
            m_count = count;
        }

        private Int32 m_id;
        public Int32 ID 
        { 
            get { return m_id; } 
        }

        private Int32 m_count;
        public Int32 Count 
        { 
            get { return m_count; } 
        }
    }

    // 2 .---------------------------------------------

    class DataAccess
    {
        public delegate void UpdateEventHandler(Object sender, UpdateEventArgs e);

        public event UpdateEventHandler OnChange;

        public void UpdateData(Int32 id, Int32 count)
        {
            if (count ==  0 ) return;

            //code to update database...

            UpdateEventArgs e = new UpdateEventArgs(id, count);

            if (OnChange != null)
                OnChange(this, e);
        }
    }


    // 3 .---------------------------------------------------------------

    class Messenger
    {
        public Messenger(DataAccess dataAccess, String[] users)
        {
            m_dataAccess = dataAccess;

            m_users = users;

            m_dataAccess.OnChange +=
                new DataAccess.UpdateEventHandler(OnChange);
        }

        private DataAccess m_dataAccess;
        private String[] m_users;

        private void OnChange(Object sender, UpdateEventArgs e)
        {
            Console.WriteLine("Product #{0} updated to {1}", e.ID, e.Count);

            foreach (String user in m_users)
                Console.WriteLine("\tSending message to {0}... ", user);
        }
    }

    // 4 .-------------------------------------------------------------------------

    class CTest
    {
        public static void Test()
        {
            String[] users = { "Vasya", "Gosha", "Petya" };

            DataAccess dataAccess = new DataAccess();
            Messenger messenger = new Messenger(dataAccess, users);

            dataAccess.UpdateData( 1 ,  3 );
            dataAccess.UpdateData( 2 ,  5 );
        }
    }