Events

using System;
namespace MyCollections
{
using System.Collections;

// 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);
}
}
}
}

namespace TestEvents
{
using MyCollections;

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;
}
}

class Test
{
// Test the ListWithChangedEvent class.
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();
listener.Detach();
}
}
}