Click here to download the sample source code for this lesson.  

ASP.NET uses an event driven programming model.  This means that in order for code to run, an event must occur that triggers or tells ASP.NET to run an event handler Method.  An event handler method is a special type of Method in an ASP.NET web page that defines the actions that will occur when a particular event occurs.  So far, throughout the lessons we have experienced one of these types of events: the click event.  We saw that if you are in the Design view for a web form and you double click on a Button control, Visual Web Developer will automatically generate an event handler Method called Button1_Click.  Basically an empty Method is generated and Visual Web Developer lets you put whatever code you want into that event handler.  Notice the naming convention that Visual Web Developer when it generates the event handler Method.  The first part of the name is the ID for the server control: Button1.  The second part of the name an underscore appended with the event type: Click.  This convention helps you as the developer know when that Method will be called by ASP.NET.  In this case, the click event fires when a user single clicks on the Button control.

ASP.NET supports different types of events for the various server controls but there are some very common events and those are the ones we will focus on in the next few lessons.  The first of these events that we will practice with is the Page Load event.  This event fires every time you navigate to an ASP.NET web page in your web browser.  When type a .aspx page in your browser’s URL window or click on a link that takes you to an .aspx page, you are triggering the Page Load event on the web server at the time the page is requested.

Create a new web form called Default3.aspx.  Drag a TextBox onto the web form and change the ID to txtHello.  Double click on a white area of the web form where there are no server controls.  Visual Web Developer should take you into the Page Load event Method Page_Load.  In there, put the following:

txtHello.Text = "Message from the Page Load event";

Go back to the Design view for Default3.aspx and drag the right side of the TextBox control to the right (about 500px) in order to make it larger; this way you will be able to read the entire message “This info was put…” when the page runs.  Run the application and notice that when Default3.aspx comes up, the message is already in the TextBox.  Now close the browser and go back to the Design view for Default3.aspx and drag a Button control to the left of the TextBox.  Change the ID property of the Button to btnPutmessage and the Text property to “Change the message”.  Double click the Button to create the event handler Method.  *Notice that the event handler is called btnPutmessage_Click.  In the code put the following:

txtHello.Text = "Message from the Button Click event";

Run the application again and this time click on the Button.  Notice that the message in the TextBox changes because ASP.NET fired the btnPutMessage_Click event when you clicked the Button.