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using System;
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);
}
}
}
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;
}
}
public static class Program {
// 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();
}
}