Using the IsPostBack Variable in .NET framework

Maker UPC Code in .NET framework Using the IsPostBack Variable

A postback occurs only when the user clicks a button or performs some other action that causes the page currently displayed by the browser to be sent back to the server The first time a page is retrieved by a user, a postback doesn t occur For instance, when the user starts the Simple Calculator application (our example from the previous chapter of this mini-book) by browsing to the application s Defaultaspx page, the page is loaded into server memory and events are processed, but a postback doesn t occur (In the case of the Simple Calculator application, a postback doesn t occur until the user clicks the Add button) As you ll see throughout this book, you ll often write code that checks to see if a postback has occurred For example, suppose you want to initialize the

Text property of a label to Good Morning! the first time a page is posted, but to change the label whenever the user posts back to the page by clicking a button; then the label should display Hello Again! instead of Good Morning! In that case, you might write Visual Basic code that looks like the following: If IsPostBack Then lblGreetingText = Hello Again! Else lblGreetingText = Good Morning! End If

If you re working in C#, the code would look more like this:

if (IsPostBack) lblGreetingText = Hello Again! ; else lblGreetingText = Good Morning! ;

Either way, the IsPostBack variable is used to determine whether a postback has occurred if it has, the label s Text property is set accordingly Careful how you use this one Using IsPostBack incorrectly can lead to all sorts of programming problems If you find that a variable or field is incorrectly reset to the same value each time the page is posted, you may need to add an if statement to set the variable or field only when IsPostBack is false You ll see many more examples of the IsPostBack variable in use throughout this book

Events are the key to ASPNET programming because most of the code you write for an ASPNET application is executed in response to events that are raised as a page is processed In essence, an ASPNET application consists of two things: A set of Web pages that contain controls that users can interact with When a user interacts with one or more of the controls on a page, an event is generated to record the interaction Code executed in response to the events generated when the user interacts with the controls on the Web pages For example, consider the Simple Calculator application presented in the previous chapter of this mini-book In practical terms, it s a Web page that

displays some labels, text boxes, a button, and some code that s executed when the user clicks the button Button-click events are among the most common types of events but by no means the only events that ASPNET pages encounter Every time a page is posted to a Web server, a series of events are processed or raised by the ASPNET server In particular, the following events are always raised every time a page is posted: Preinit: This is the first event raised as a part of page initialization It occurs before ASPNET initializes the page It allows you to change properties that may affect the way the page is initialized Init: This event occurs after ASPNET has initialized the page; it allows you to add additional initialization code Load: This event kicks in after the controls have been initialized with their correct values Control events: Any events that result from the user interacting with controls, such as when the user clicks a button or selects an item from a drop-down list, are raised after the Load event For example, if the user clicks a button, the Click event will be raised after the Load event Note that it s entirely possible (and quite common) for more than one control event to occur each time a page is posted For example, if a user selects an item from a drop-down list, clicks a check box to select it, then clicks a button, the page is posted when the button is clicked Then three events are raised after the Load event: one to indicate that the user has selected an item from the drop-down list, a second to indicate that the user has clicked the check box, and a third to indicate that the user has clicked the button PreRender: This event is raised after all of the control events have been raised but before the HTML that will be sent back to the browser is generated Unload: This event is raised when the page has been sent back to the browser The process of connecting a method (or, as it s called in VBNET, a Sub procedure) to an event is called wiring There are two ways to wire events, depending on whether you re working in Visual Basic or C# The next two sections describe the two techniques (Hint: No screwdriver required)