Understanding Events in C#

Introduction

Events are one of the core and important concepts of the C# .Net Programming environment and frankly speaking, sometimes it's hard to understand them without a proper explanation and example.

So I thought of writing this article to make things easier for learners and beginners.

Our Topic

An event in very simple terms is an action or occurrence, such as clicks, key press, mouse movements, or system generated notifications. Applications can respond to events when they occur. Events are messages sent by the object to indicate the occurrence of the event. Events are an effective method of inter-process communication. They are useful for an object because they provide signal state changes that may be valuable to a client of the object.

If the preceding sentences were tough to understand then let's make it simpler. If a button on a form is clicked by a user then an event is fired. If a user types something into a TextBox then keys are pressed and hence an event is fired and so on.

The following figure is the generalized representation that explains events and event handling.



In C#, delegates are used with events to implement event handling. The .NET Framework event model uses delegates to bind notifications with methods known as event handlers. When an event is generated, the delegate calls the associated event handler.

Investigating .NET Windows Application Button Click Event

Just open your Visual Studio and create a Windows application. You'll get a form in your Windows application project named Form1.cs. Add a simple button to that form, just drag and drop. In the properties panel of that button, bind the onclick event of that button with an event in the code behind and show some text on its click. You can browse the attached source code for understanding.

Now when you run the application, your form will be shown with just one button, click that button and see what happens.



As we can see the button on the form was clicked by the user and hence the button click event was fired that was handled by the delegate that in turn called the button_click method (event handler method) that showed our Message Box.

In the preceding example of an event declaration, delegate declaration and event handler declaration, all is done by .NET. We just need to write code handle to our event. In this case the code to display the message box.

Behind the curtain

If we investigate Form1.Designer.cs and follow the step shown in the figure below we can easily determine the event keyword and delegate keyword and hence determine their definition.

Since we haven't seen the definition syntax it might look alien to you but we will get to it soon. For now just follow the procedure in the figure.

Step 1

Open Form1.Designer.Cs from Solution Explorer.



Step 2

In Form1.Designer.Cs Click event and EventHandler delegate.



Step 3

Double-click this.button1. Click and press F12 to see the Click Event definition, similarly double-click System.EventHandler and Press F12 to see the EventHandler Delegate definition.

The following shows the event's definition:



The following shows the delegate's definition:



Publisher-Subscriber model

The events are declared and raised in a class and associated with the event handlers using delegates within the same class or other classes. Events are part of a class and the same class is used to publish its events. The other classes can, however, accept these events, or in other words can subscribe to these events. Events use the publisher and subscriber model.

A publisher is an object that contains the definition of the event and the delegate. The association of the event with the delegate is also specified in the publisher class. The object of the publisher class invokes the event that is notified to the other objects.

A subscriber is an object that wants to accept the event and provide a handler to the event. The delegate of the publisher class invokes the method of the subscriber class. This method in the subscriber class is the event handler. The publisher and subscriber model implementation can be defined by the same class.

The following figure shows the mechanism used by the publisher and subscriber objects.



Getting your hand dirty

Let's get our hand dirty by building our own event handling example. In the example below we will see how to define our customized event and how to raise it and how to handle it by our own customized event handler.

In our simple example we'll build a console application for a bank in which the event TransactionMade is raised whenever the customer makes a transaction and in response a notification is sent to him.

Let's do some serious coding now.

First we define our class Account.

We can add a constructor to initialize our variable int BalanceAmount that will hold the account balance in our class.