Environment and Life Cycle
ASP.NET provides an abstraction layer on top of HTTP on which the web applications are built. It provides high-level entities such as classes and components within an object-oriented paradigm.
The key development tool for building ASP.NET applications and front ends is Visual Studio. In this tutorial, we work with Visual Studio 2008.
Visual Studio is an integrated development environment for writing, compiling, and debugging the code. It provides a complete set of development tools for building ASP.NET web applications, web services, desktop applications, and mobile applications.
The Visual Studio IDE
The new project window allows choosing an application template from the available templates.
When you start a new web site, ASP.NET provides the starting folders and files for the site, including two files for the first web form of the site.
The file named Default.aspx contains the HTML and asp code that defines the form, and the file named Default.aspx.cs (for C# coding) or the file named Default.aspx.vb (for VB coding) contains the code in the language you have chosen and this code is responsible for the actions performed on a form.
The primary window in the Visual Studio IDE is the Web Forms Designer window. Other supporting windows are the Toolbox, the Solution Explorer, and the Properties window. You use the designer to design a web form, to add code to the control on the form so that the form works according to your need, you use the code editor.
Working with Views and Windows
You can work with windows in the following ways:
To change the Web Forms Designer from one view to another, click on the Design or source button.
To close a window, click on the close button on the upper right corner and to redisplay, select it from the View menu.
To hide a window, click on its Auto Hide button. The window then changes into a tab. To display again, click the Auto Hide button again.
To change the size of a window, just drag it.
Adding Folders and Files to your Website
When a new web form is created, Visual Studio automatically generates the starting HTML for the form and displays it in Source view of the web forms designer. The Solution Explorer is used to add any other files, folders or any existing item on the web site.
To add a standard folder, right-click on the project or folder under which you are going to add the folder in the Solution Explorer and choose New Folder.
To add an ASP.NET folder, right-click on the project in the Solution Explorer and select the folder from the list.
To add an existing item to the site, right-click on the project or folder under which you are going to add the item in the Solution Explorer and select from the dialog box.
Projects and Solutions
A typical ASP.NET application consists of many items: the web content files (.aspx), source files (.cs files), assemblies (.dll and .exe files), data source files (.mdb files), references, icons, user controls and miscellaneous other files and folders. All these files that make up the website are contained in a Solution.
When a new website is created. VB2008 automatically creates the solution and displays it in the solution explorer.
Solutions may contain one or more projects. A project contains content files, source files, and other files like data sources and image files. Generally, the contents of a project are compiled into an assembly as an executable file (.exe) or a dynamic link library (.dll) file.
Typically a project contains the following content files:
- Page file (.aspx)
- User control (.ascx)
- Web service (.asmx)
- Master page (.master)
- Site map (.sitemap)
- Website configuration file (.config)
Building and Running a Project
You can execute an application by:
- Selecting Start
- Selecting Start Without Debugging from the Debug menu,
- pressing F5
The program is built meaning, the .exe or the .dll files are generated by selecting a command from the Build menu.
ASP.NET life cycle specifies, how:
- ASP.NET processes pages to produce dynamic output
- The application and its pages are instantiated and processed
- ASP.NET compiles the pages dynamically
The ASP.NET life cycle could be divided into two groups:
- Application Life Cycle
- Page Life Cycle
ASP.NET Application Life Cycle
The application life cycle has the following stages:
User makes a request for accessing application resource, a page. Browser sends this request to the web server.
A unified pipeline receives the first request and the following events take place:
An object of the class ApplicationManager is created.
An object of the class HostingEnvironment is created to provide information regarding the resources.
Top level items in the application are compiled.
Response objects are created. The application objects such as HttpContext, HttpRequest and HttpResponse are created and initialized.
An instance of the HttpApplication object is created and assigned to the request.
The request is processed by the HttpApplication class. Different events are raised by this class for processing the request.
ASP.NET Page Life Cycle
When a page is requested, it is loaded into the server memory, processed, and sent to the browser. Then it is unloaded from the memory. At each of these steps, methods and events are available, which could be overridden according to the need of the application. In other words, you can write your own code to override the default code.
The Page class creates a hierarchical tree of all the controls on the page. All the components on the page, except the directives, are part of this control tree. You can see the control tree by adding trace= "true" to the page directive. We will cover page directives and tracing under 'directives' and 'event handling'.
The page life cycle phases are:
- Instantiation of the controls on the page
- Restoration and maintenance of the state
- Execution of the event handler codes
- Page rendering
Understanding the page cycle helps in writing codes for making some specific thing happen at any stage of the page life cycle. It also helps in writing custom controls and initializing them at right time, populate their properties with view-state data and run control behavior code.
Following are the different stages of an ASP.NET page:
Page request - When ASP.NET gets a page request, it decides whether to parse and compile the page, or there would be a cached version of the page; accordingly the response is sent.
Starting of page life cycle - At this stage, the Request and Response objects are set. If the request is an old request or post back, the IsPostBack property of the page is set to true. The UICulture property of the page is also set.
Page initialization - At this stage, the controls on the page are assigned unique ID by setting the UniqueID property and the themes are applied. For a new request, postback data is loaded and the control properties are restored to the view-state values.
Page load - At this stage, control properties are set using the view state and control state values.
Validation - Validate method of the validation control is called and on its successful execution, the IsValid property of the page is set to true.
Postback event handling - If the request is a postback (old request), the related event handler is invoked.
Page rendering - At this stage, view state for the page and all controls are saved. The page calls the Render method for each control and the output of rendering is written to the OutputStream class of the Response property of page.
Unload - The rendered page is sent to the client and page properties, such as Response and Request, are unloaded and all cleanup done.
ASP.NET Page Life Cycle Events
At each stage of the page life cycle, the page raises some events, which could be coded. An event handler is basically a function or subroutine, bound to the event, using declarative attributes such as Onclick or handle.
Following are the page life cycle events:
PreInit - PreInit is the first event in page life cycle. It checks the IsPostBack property and determines whether the page is a postback. It sets the themes and master pages, creates dynamic controls, and gets and sets profile property values. This event can be handled by overloading the OnPreInit method or creating a Page_PreInit handler.
Init - Init event initializes the control property and the control tree is built. This event can be handled by overloading the OnInit method or creating a Page_Init handler.
InitComplete - InitComplete event allows tracking of view state. All the controls turn on view-state tracking.
LoadViewState - LoadViewState event allows loading view state information into the controls.
LoadPostData - During this phase, the contents of all the input fields are defined with the <form> tag are processed.
PreLoad - PreLoad occurs before the post back data is loaded in the controls. This event can be handled by overloading the OnPreLoad method or creating a Page_PreLoad handler.
Load - The Load event is raised for the page first and then recursively for all child controls. The controls in the control tree are created. This event can be handled by overloading the OnLoad method or creating a Page_Load handler.
LoadComplete - The loading process is completed, control event handlers are run, and page validation takes place. This event can be handled by overloading the OnLoadComplete method or creating a Page_LoadComplete handler
PreRender - The PreRender event occurs just before the output is rendered. By handling this event, pages and controls can perform any updates before the output is rendered.
PreRenderComplete - As the PreRender event is recursively fired for all child controls, this event ensures the completion of the pre-rendering phase.
SaveStateComplete - State of control on the page is saved. Personalization, control state and view state information is saved. The HTML markup is generated. This stage can be handled by overriding the Render method or creating a Page_Render handler.
UnLoad - The UnLoad phase is the last phase of the page life cycle. It raises the UnLoad event for all controls recursively and lastly for the page itself. Final cleanup is done and all resources and references, such as database connections, are freed. This event can be handled by modifying the OnUnLoad method or creating a Page_UnLoad handler.