Beginning Web Development, Silverlight, and ASP.NET AJAX

58
CHAPTER 3 ■ WEB FORMS WITH ASP.NET
handle the HTTP-POST from the form. Other initialization, such as configuring controls,
retrieving database information, and so on, is usually done at this stage so that when the
page is rendered, it is provided with all the information the user needs.
It’s important to note that Page_Load always fires, whether this is the first time the
page has loaded or it is a postback. With the latter, you might expect it not to load,
because the page is already loaded, but it does. Due to the stateless nature of web applications,
each postback is treated as a new request to the page, and view state is used to
maintain state between calls, as discussed earlier.
As a developer, you may need to discern whether you are in a postback situation or
an initial load situation, and ASP.NET provides this information for you via the IsPostBack
property on the Page object.
Thus, Page.IsPostBack will be true upon a postback and false on the first load. If
there are actions that you only want to perform on either case, you can gate them inside
an if(Page.IsPostBack) clause.
Performing Validation
An important aspect of any web application is verifying that the data the user has input is
valid. This ensures that your application is secure (e.g., so that no one is able to attempt
Structured Query Language [SQL] injection attacks) and the user experience is optimized;
only appropriate data is sent to the server and returned from the server. ASP.NET
introduces a suite of self-sufficient validation controls that can be used to ensure that the
content of your page meets validation rules. These are very useful controls, relieving you
from writing reams of validation code to ensure that the correct data has been input.
Performing Event Handling
Earlier we saw how events are handled on a page. Some events provide an automatic
postback that triggers an event handler. Others—such as a list box selection—aren’t handled
immediately, unless they have their AutoPostBack property set to true. At this stage
of the life cycle, ASP.NET invokes the event handlers for each of the events that were
tripped. So, if your page has a number of events before the postback occurs—for example,
if a number of lists are used with no AutoPostBack, and the user selects items on them
before pressing a button on the page—then each of these events will have their handler
called at this stage.
In addition, a number of page-level events need to be performed. In the scenario in
which two lists do not have AutoPostBack set to true, and there’s no button that causes the
postback, events will be fired in the following order:
1. Page_Init
2. Page_Load