This tutorial shows how to declare, invoke, and hook up to events in C#.
Tutorial
An event in C# is a way for a class to provide notifications to clients of that class when some interesting thing happens to an object. The most familiar use for events is in graphical user interfaces; typically, the classes that represent controls in the interface have events that are notified when the user does something to the control (for example, click a button).
Events, however, need not be used only for graphical interfaces. Events provide a generally useful way for objects to signal state changes that may be useful to clients of that object. Events are an important building block for creating classes that can be reused in a large number of different programs.
Events are declared using delegates. If you have not yet studied the Delegates Tutorial, you should do so before continuing. Recall that a delegate object encapsulates a method so that it can be called anonymously. An event is a way for a class to allow clients to give it delegates to methods that should be called when the event occurs. When the event occurs, the delegate(s) given to it by its clients are invoked.
In addition to the examples on declaring, invoking, and hooking up to events, this tutorial also introduces the following topics:
Example
The following simple example shows a class,ListWithChangedEvent, which is similar to the standard ArrayList class, but also invokes a Changed event whenever the contents of the list change. Such a general-purpose class could be used in numerous ways in a large program.
For example, a word processor might maintain a list of the open documents. Whenever this list changes, many different objects in the word processor might need to be notified so that the user interface could be updated. By using events, the code that maintains the list of documents doesn’t need to know who needs to be notified — once the list of documents is changed, the event is automatically invoked and every object that needs to be notified is correctly notified. By using events, the modularity of the program is increased.
// Events Tutorial – Chimomousing System;using System.Collections;namespace EventsTutorial{// 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.class ListWithChangedEvent : ArrayList{// An event that clients can use to be notified whenever the elements of list change.public event ChangedEventHandler Changed;// Invoke the Changed event; called whenever list changes.private 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);}}}}// Events Tutorial – Chimomousing System;namespace EventsTutorial{class EventListener{private ListWithChangedEvent listWithChangedEvent;public EventListener(ListWithChangedEvent list){listWithChangedEvent = list;// Add "ListChanged" to the Changed event on "listWithChangedEvent".listWithChangedEvent.Changed += 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 listWithChangedEvent.listWithChangedEvent.Changed -= ListChanged;listWithChangedEvent = null;}}}// Events Tutorial – Chimomonamespace EventsTutorial{static class Program{// 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();}}}// Output:/*This is called when the event fires.This is called when the event fires.*/Code DiscussionDeclaring an eventTo declare an event inside a class, first a delegate type for the event must be declared, if none is already declared.public delegate void ChangedEventHandler(object sender, EventArgs e);
The delegate type defines the set of arguments that are passed to the method that handles the event. Multiple events can share the same delegate type, so this step is only necessary if no suitable delegate type has already been declared.
Next, the event itself is declared.
public event ChangedEventHandler Changed;
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