Visual Basic.NET How to Program (PDF)

Chapter 20 ASP .NET, Web Forms and Web Controls 1023
The asp: tag prefix in the declaration of a control indicates that the control is an ASP .NET Web
control.
Each Web control maps to a corresponding HTML element.
The same Web control can map to different HTML elements, depending on the client browser and
the Web control’s property settings.
Namespace System.Web contains classes that manage client requests and server responses.
Namespace System.Web.UI contains classes for the creation of Web-based applications and
controls.
Class Page defines a standard Web page, providing event handlers and objects necessary for creating
Web-based applications. All code-behind classes for ASPX forms inherit from class Page.
Class Control is the base class that provides common functionality for all Web controls.
Namespace System.Web.UI.WebControls contains Web controls employed in the design
of the page’s user interface.
Method Page_Init is called when the Init event is raised. This event indicates that the page
is ready to be initialized.
The Load event is raised when the page loads. (This event occurs after all the Web controls on
the page have been initialized and loaded.)
When a client requests an ASPX file, a class is created behind the scenes that contains both the
visual aspect of our page (defined in the ASPX file) and the logic of our page (defined in the codebehind
file). This new class inherits from Page. The first time that our Web page is requested, this
class will be compiled, and an instance will be created. This instance represents our page—it will
create the HTML that is sent to the client. The assembly created from our compiled class will be
placed in the project’s bin directory.
Changes to the Web application can be detected by the runtime, and the project is recompiled to
reflect the altered content.
The Page_Load event handler is usually used to execute any processing that is necessary to restore
data from previous requests.
After Page_Load has finished executing, the page processes any events raised by the page’s controls.
When a Web Form object is ready for garbage collection, an Unload event is raised. Event handler
Page_Unload is inherited from class Page and contains any code that releases resources.
A form is a mechanism for collecting user information and sending it to the Web server.
HTML forms can contain visual and nonvisual components. Visual components include clickable
buttons and other graphical user interface components with which users interact.
Nonvisual components in an HTML form, called hidden inputs, store any data that the document
author specifies.
The name localhost indicates that the client and server reside on the same machine. If the Web
server were located on a different machine, localhost would be replaced with the appropriate
IP address or hostname.
The Web-Form designer can display HTML mode, allowing the programmer to view the markup
that represents the user interface of this page. The Design mode allows the programmer to view
the page as it will look and modify it using the drag-and-drop technique.
The PageLayout property determines how controls are arranged on the form.
By default, property PageLayout is set to GridLayout, which means that all controls remain
exactly where they are dropped on the Web Form. This is called absolute positioning.