VB.NET Event handling in C#

Event Handling in C#

.NET is a world of events. Everything happens on a particular event. Today I will try to explain you events in simple language so that you can use it effectively in your code.

Events can not exist without delegates. A delegate in C# is similar to a function pointer in C or C++. Using a delegate allows the programmer to encapsulate a reference to a method inside a delegate object. The delegate object can then be passed to code which can call the referenced method, without having to know at compile time which method will be invoked.

Whenever a declared event occurs it publishes itself and all the subscribers to that event receives notification. There could be n subscribers for an event.

Things will get clear as we proceed with the example. Follow these steps

1) Create a windows application and add two windows forms.

2) Create a following class in same project.

using System;

using System.Threading;

namespace EventHandling

{

/// <summary>

/// Summary description for EventDeclaration.

/// </summary>
public class EventDeclaration

{

public delegate void RespondAfterOneMinute();
public event RespondAfterOneMinute OnRespondAfterOneMinute;

public EventDeclaration()

{

Thread trd = new Thread(new ThreadStart(StartCountingMinutes));

trd.Priority = ThreadPriority.Lowest;

trd.IsBackground= true;

trd.Start();

}

public void StartCountingMinutes()

{

while(true)

{
Thread.Sleep(6000);

if(OnRespondAfterOneMinute != null)

{

OnRespondAfterOneMinute();

}

In this class we have declared a delegate and an event.

public delegate void RespondAfterOneMinute();

public event RespondAfterOneMinute OnRespondAfterOneMinute;

This event will publish itself after every 6 sec. Now in the constructor of this class we have created a new thread in background which will take care of publishing this event.

if(OnRespondAfterOneMinute != null).

This statement will check after every 6 sec whether there are any subscribers present for this event or not. If subscribers are present then it will notify them and will tell them that the event you are interested in have occurred.

Now let see what we need to do on two windows forms that we have created. These are nothing but the subscribers for our event.

We will declare an object of the class EventDeclaration

EventDeclaration objEvent = new EventDeclaration();

Now on click of Register Event button we will write following code

objEvent.OnRespondAfterOneMinute +=new EventHandling.EventDeclaration.RespondAfterOneMinute (objEvent_OnRespondAfteroneMinute);

It should be noted that the method we are going to execute on this event should have signature exactly similar to that of the delegate we have declared. So this function should return void and should not have any parameters in it.

private void objEvent_OnRespondAfterOneMinute()

{
txt_Log.Text += "Got Message at:" + DateTime.Now.ToString();
txt_Log.Text += "\t\t\t";

}

Repeat this piece of code for listener 2. Now when you will execute this code you can observe that after every 6 seconds all the listeners who have registered themselves for this event are getting notification.

You can unregister an event by simply writing following code

objEvent.OnRespondAfterOneMinute -=new EventHandling.EventDeclaration.RespondAfterOneMinute (objEvent_OnRespondAfterOneMinute);

I am attaching complete code of the sample we just discussed. You can go through it and that will give you a better idea.