Events using delegates (c#) – An additional example

A former colleague wrote an excellent blog post about using Events with Delegates, in C#. He is one of the smartest people I know and very active in the community, with a deep understanding of a dozen technologies with never ending passion for technology. I haven't found a simpler example on the web yet. I wanted to add an additional real world example written in ASP.net – The example is of a News paper (Publisher) and Customers (Subscribers). Here it is:

    1: /* Event delegation works on the Publisher <-> Subscriber Model. The Publisher exposes events that clients who are interested can subscribe

    2:  * to and when an event occurs, the subscribers are notified.

    3:  * If we were to consider a common real world example, we could think of implementing a Newspapaer and its Subscibers

    4:  * The code below implements the Newspaper as the Publisher and its Subscribers as customers who subscribe to be notified of breaking news.

    5:  */

    6:  

    7: using System;

    8: using System.Web;

    9:  

   10: namespace Example4

   11: {

   12:     public partial class EventsDemo : System.Web.UI.Page

   13:     { 

   14:         protected void Page_Load(object sender, EventArgs e)

   15:         {

   16:             //We will first instantiate our Publisher - Which is our news paper - TimesOfIndia

   17:             TimesOfIndia TOI = new TimesOfIndia();

   18:  

   19:             //Add subscribers

   20:             TOICustomer Sudeepg = new TOICustomer(TOI, "Sudeepg");

   21:             TOICustomer Jack    = new TOICustomer(TOI, "Jack");

   22:             TOICustomer George  = new TOICustomer(TOI, "George");

   23:  

   24:             //Times now will get the breaking news for us

   25:             TOI.getBreakingNews();

   26:         }

   27:     }

   28:  

   29:     //This is our public delegate that will be hooked up by our subscribers.

   30:     public delegate void BreakingNewsEventHandler(object sender, BreakingNewsEventArgs e);

   31:  

   32:     public class TimesOfIndia  // Times of india news paper is our publisher.

   33:     {

   34:         public event BreakingNewsEventHandler breakingNewsNotifyList;

   35:  

   36:         public void getBreakingNews()

   37:         {

   38:             string BreakingNews = "Annual Sale - Best deals on electronic devices!";   //Sample breaking news

   39:  

   40:             //Display the breaking news - NOTE: The Marquee tag only works in IE. Remove it for other browsers.

   41:             HttpContext.Current.Response.Write("<h1><marquee> Current Breaking News : " + BreakingNews.ToString() + "</marquee></h1><hr />");

   42:  

   43:             //Create and add event args

   44:             BreakingNewsEventArgs e = new BreakingNewsEventArgs(BreakingNews);

   45:  

   46:             //We need to invoke the notification, ONLY if there are subscribers to breaking news notification. That's why we check for NULL.

   47:             if (breakingNewsNotifyList != null)

   48:                 breakingNewsNotifyList(this, e);

   49:  

   50:         }

   51:     }

   52:  

   53:     public class BreakingNewsEventArgs : EventArgs      //Class used to pass data to event consumers when sending notification.

   54:     {

   55:         public BreakingNewsEventArgs(string BreakingNews)

   56:         {

   57:             this.m_BreakingNews = BreakingNews;         //Save the breaking news into a property.

   58:         }

   59:  

   60:         string m_BreakingNews = "";

   61:         public string BreakingNews      //Data we are passing to consumers is the breaking news itself (Stored as property).

   62:         {

   63:             get { return m_BreakingNews; }

   64:         }

   65:     }

   66:  

   67:  

   68:     public class TOICustomer        //Instances of this class will be the subscribers of Times of India (Publisher).

   69:     {

   70:         private TimesOfIndia TOI;               //Copy of the newsPaper

   71:         private string m_CustomerName = "";     //Store the customer name

   72:  

   73:         public TOICustomer(TimesOfIndia TOICopy, string customerName)

   74:         {

   75:             this.TOI = TOICopy;

   76:             m_CustomerName = customerName;

   77:             TOI.breakingNewsNotifyList += new BreakingNewsEventHandler(OnRecvBreakingNews);  //Subscribe to breaking news notification

   78:         }

   79:  

   80:         //This method will be invoked upon a breaking news Notification.

   81:         private void OnRecvBreakingNews(object sender, BreakingNewsEventArgs e)

   82:         {

   83:             HttpContext.Current.Response.Write(m_CustomerName + " Acknowledging breaking news : " + e.BreakingNews.ToString() + "<br />");

   84:         }

   85:     }

   86: }

Comments

• Anonymous
  January 21, 2010
  Lot of thanks for sharing your colleague's post. Really it is very for me