C# Events Explained - Event Without an Event
(Page 6 of 6 )
I have modified the example to remove the event and to use the delegate only to fire the event. Here's the code:
using System;
using System.Collections;
namespace Events
{
public delegate void AddBookEventHandler(object source, BookEventArgs e);
public class BookShop
{
private ArrayList books;
private string name;
public BookShop(string name)
{
books = new ArrayList();
this.name = name;
}
protected void OnAddBook(BookEventArgs e)
{
if(AddBook != null)
AddBook(this, e);
}
public void AddNewBook(Book book)
{
this.books.Add(book);
BookEventArgs e = new BookEventArgs(book);
OnAddBook(e);
}
private AddBookEventHandler addBook;
public AddBookEventHandler AddBook
{
get{return addBook;}
set {addBook = value;}
}
}
public class Book
{
private string title;
private string isbn;
private decimal price;
public Book(string title, string isbn, decimal price)
{
this.title = title;
this.isbn = isbn;
this.price = price;
}
public string Title
{
get{return this.title;}
}
public string ISBN
{
get{return this.isbn;}
}
public decimal Price
{
get{return this.price;}
}
}
public class BookEventArgs: EventArgs
{
private Book book;
public BookEventArgs(Book book)
{
this.book = book;
}
public Book Book
{
get { return this.book;}
}
}
public class Notifier
{
public void instance_AddBook(object source, BookEventArgs e)
{
Console.WriteLine("A Message has been sent to the Membersn" + " about the book '{0}' with the ISBN '{1}'n", e.Book.Title, e.Book.ISBN);
//((BookShop)source).AddBook = null;
}
}
public class Sys
{
public static void Main()
{
BookShop bookShop = new BookShop(".NET Bookshop");
Book b1 = new Book("The Right Way","123456789", 19.90m);
Book b2 = new Book("The .NET Stuff","987654321", 29.90m);
Notifier notify = new Notifier();
bookShop.AddBook =
new AddBookEventHandler(notify.instance_AddBook);
bookShop.AddNewBook(b1);
bookShop.AddNewBook(b2);
Console.ReadLine();
}
}
}
Run the code and you will have the same results. Now uncomment the last statement in the instance_AddBook() method and run the application again.
Only the event-like mechanism fires for the first book added to the bookshop; it didn't fire for the next time a book is added. We were able to set the value of the Multicast delegate to null in the watcher class by casting the source object (which is the BookShop instance that fired the event) to BookShop, then assigned a null value to the property AddBook. By assigning a null value to the AddBook property we remove the other subscribed event handler methods, causing the event to stop functioning. Also now you can use the = instead of += operator to assign a delegate to the multicast delegate, which also is not valid because the event should adds on its multicast delegate, not replace it.
What we have done in the above application is remove the event from the BookShop class then create a private field of delegate type AddBookEventHandler and its public property. This property is used to assign a AddBookEventHandler delegate instance to the private field addBook. The method OnAddBook() raises the event by calling the delegate through the AddBook property. In the Sys class we assign a new AddBookEventHandler delegate instance to the AddBook property using the Assignment operator. The event is raised but because AddBook is a public property we have the ability to assign any new value to it outside the BookShop instance as we did in the event handler method and set null value to this property. So when we use this mechanism we define AddBook as a property.
But with the event model we define it as an event which has restrictions
As you can see, the event is very similar to the property that we created but with restrictions. To understand this better take the following statement and place it at the end of the Notifier class' event handler method of the event example (not the event without an event section) then try to compile the code.
((BookShop)source).AddBook = null;
The code will not compile and you will get the following error:
It says that you can't use the AddBook event except as the left hand side of += or -= operations outside the BookShop. I hope that by now you understand how events are created and what the difference is between using an event and a delegate.
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