namespace TestCollections
{
    // A delegate type for hooking up change notifications.
    public delegate void ChangedEventHandler(object sender, System.EventArgs e);

    // A class that works just like ArrayList, but sends event
    // notifications whenever the list changes.
    public class ListWithChangedEvent : System.Collections.ArrayList
    {
        // An event that clients can use to be notified whenever the
        // elements of the list change.
        public event ChangedEventHandler Changed;

        // Invoke the Changed event; called whenever list changes
        protected virtual void OnChanged(System.EventArgs e)
        {
            if (Changed != null)
            {
                Changed(this, e);
            }
        }

        // Override some of the methods that can change the list;
        // invoke event after each
        public override int Add(object value)
        {
            int i = base.Add(value);
            OnChanged(System.EventArgs.Empty);
            return i;
        }

        public override void Clear()
        {
            base.Clear();
            OnChanged(System.EventArgs.Empty);
        }

        public override object this[int index]
        {
            set
            {
                base[index] = value;
                OnChanged(System.EventArgs.Empty);
            }
        }
    }
}

namespace TestEvents
{
    using TestCollections;

    class EventListener
    {
        private ListWithChangedEvent m_list;

        public EventListener(ListWithChangedEvent list)
        {
            m_list = list;

            // Add "ListChanged" to the Changed event on m_list:
            m_list.Changed += new ChangedEventHandler(ListChanged);
        }

        // This will be called whenever the list changes.
        private void ListChanged(object sender, System.EventArgs e)
        {
            System.Console.WriteLine("This is called when the event fires.");
        }

        public void Detach()
        {
            // Detach the event and delete the list
            m_list.Changed -= new ChangedEventHandler(ListChanged);
            m_list = null;
        }
    }

    class Test
    {
        // Test the ListWithChangedEvent class.
        static void Main()
        {
            // Create a new list.
            ListWithChangedEvent list = new ListWithChangedEvent();

            // Create a class that listens to the list's change event.
            EventListener listener = new EventListener(list);

            // Add and remove items from the list.
            list.Add("item 1");
            list.Clear();
            listener.Detach();
        }
    }
}