using System;

using System.Collections;

​

public class Program

{

    public 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();

        // detach the listener

        listener.Detach();

    }

}

​

// A delegate type for hooking up change notifications.

public delegate void ChangedEventHandler(object sender, EventArgs e);

​

​

// A class that works just like ArrayList, but sends event

// notifications whenever the list changes.

public class ListWithChangedEvent : 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(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(EventArgs.Empty);

        return i;

    }

​

    public override void Clear()

    {

        base.Clear();

        OnChanged(EventArgs.Empty);

    }

​

    public override object this[int index]

    {

        set

        {

            base[index] = value;

            OnChanged(EventArgs.Empty);

        }

    }

}

​

class EventListener

{

    private ListWithChangedEvent List;

    public EventListener(ListWithChangedEvent list)

    {

        List = list;

        // Add "ListChanged" to the Changed event on "List".

        List.Changed += new ChangedEventHandler(ListChanged);

    }

​

    // This will be called whenever the list changes.

    private void ListChanged(object sender, EventArgs e)

    {

        Console.WriteLine("This is called when the event fires.");

    }

​

    public void Detach()

    {

        // Detach the event and delete the list

        List.Changed -= new ChangedEventHandler(ListChanged);

        List = null;

    }

}