ASP.NET Page Life Cycle

This article describes the ASP.NET PageLife Cycle from the moment the URL is hit from the web browser till the HTML code is generated and sent to the web browser.

http://www.codeproject.com/KB/aspnet/PageLifeCycle.aspx

Introduction

This article describes the life cycle of the page from the moment the URL is hit from the web browser till the HTML code is generated and sent to the web browser. Let us start by looking at some keywords that are involved in the life cycle of the page.

Background

IIS: IIS (Internet Information Server) is a complete Web server that makes it possible to quickly and easily deploy powerful Web sites and applications. It is the default web server used with .NET. When a Web server (for ASP.NET applications,
typically IIS) receives a request, it examines the file-name extension of the requested file, determines which ISAPI extension should handle the request, and then passes the request to the appropriate ISAPI extension. (By default, ASP.NET handles file name
extensions that have been mapped to it, such as .aspx, .ascx,
.ashx, and .asmx.)

Note:

If a file name extension has not been mapped to ASP.NET, ASP.NET will not receive the request. It will be handled by the IIS. The requested page/image/file is returned without any processing.
If you create a custom handler to service a particular file name extension, you must map the extension to ASP.NET in IIS and also register the handler in your application's Web.config file.
ASPNET_ISAPI.DLL: This DLL is the ISAPI extension provided with ASP.NET to process the web page requests. IIS loads this DLL and sends the page request to this DLL. This DLL loads the

HTTPRuntime

for further processing.

ASPNET_WP.EXE: Each worker process (ASPNET_WP.EXE) contains an Application Pool. Each Application Pool can contain any number of Applications. Application Pool is also called as AppDomain. When a web page is requested, IIS looks
for the application pool under which the current application is running and forwards the request to the respective worker process.

HTTP Pipeline: HTTP Pipeline is the general-purpose framework for server-side HTTP programming that serves as the foundation for ASP.NET pages as well as Web Services. All the stages involved from creating HTTP Runtime to HTTP Handler is
called HTTP Pipeline.

HTTP Runtime: Each AppDomain has its own instance of the

HttpRuntime

class—the entry point in the pipeline. The

HttpRuntime

object initializes a number of internal objects that will help carry the request out. The

HttpRuntime

creates the context for the request and fills it up with any HTTP information specific to the request. The context is represented by an instance of the

HttpContext

class. Another helper object that gets created at such an early stage of the HTTP runtime setup is the text writer—to contain the response text for the browser. The text writer is an instance of the

HttpWriter

class and is the object that actually buffers any text programmatically sent out by the code in the page. Once the HTTP runtime is initialized, it finds an application object to fulfill the request. The

HttpRuntime

object examines the request and figures out which application it was sent to (from the pipeline's perspective, a virtual directory is an application).

HTTP Context: This is created by HTTP Runtime. The

HttpContext

class contains objects that are specific to the current page request, such as the

HttpRequest

and

HttpResponse

objects. You can use this class to share information between pages. It can be accessed with

Page.Context

property in the code.


HTTP Request: Provides access to the current page request, including the request headers, cookies, client certificate, query string, and so on. You can use this class to read what the browser has sent. It can be accessed with

Page.Request

property in the code.

HTTP Response: Provides access to the output stream for the current page. You can use this class to inject text into the page, to write cookies, and more. It can be accessed with

Page.Response

property in the code.

HTTP Application: An application object is an instance of the

HttpApplication

class—the class behind the global.asax file.

HTTPRuntime

uses

HttpApplicationFactory

to create the

HTTPApplication

object. The main task accomplished by the HTTP application manager is finding out the class that will actually handle the request. When the request is for an.aspx resource, the handler is a page handler—namely, an instance of a class that inherits
from

Page

. The association between types of resources and types of handlers is stored in the configuration file of the application. More exactly, the default set of mappings is defined in the

<httpHandlers>

section of themachine.config file. However, the application can customize the list of its own HTTP handlers in the localweb.config file. The line below illustrates the code that defines the HTTP handler for.aspx resources.


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<add verb="*" path="*.aspx" type="System.Web.UI.PageHandlerFactory"/>

HttpApplicationFactory: Its main task consists of using the URL information to find a match between the virtual directory of the URL and a pooled

HttpApplication

object.

HTTP Module: An HTTP module is an assembly that is called on every request that is made to your application. HTTP modules are called as part of the ASP.NET request pipeline and have access to life-cycle events throughout the request. HTTP
modules let you examine incoming and outgoing requests and take action based on the request. They also let you examine the outgoing response and modify it. ASP.NET uses modules to implement various application features, which include forms authentication,
caching, session state, and client script services. In each case, when those services are enabled, the module is called as part of a request and performs tasks that are outside the scope of any single page request. Modules can consume application events and
can raise events that can be handled in the Global.asax file.

HTTP Handler: An ASP.NET HTTP handler is the process that runs in response to a request that is made to an ASP.NET Web application. The most common handler is an ASP.NET page handler that processes.aspx files. When users request
a .aspx file, the request is processed by the page handler. We can write our own handler and handler factory if we want to handle the page request in a different manner.

Note: HTTP modules differ from HTTP handlers. An HTTP handler returns a response to a request that is identified by a file name extension or family of file name extensions. In contrast, an HTTP module is invoked for all requests and
responses. It subscribes to event notifications in the request pipeline and lets you run code in registered event handlers. The tasks that a module is used for are general to an application and to all requests for resources in the application.

Life Cycle of Page

Web page request comes from browser.
IIS maps the ASP.NET file extensions to ASPNET_ISAPI.DLL, an ISAPI extension provided with ASP.NET.
ASPNET_ISAPI.DLL forwards the request to the ASP.NET worker process (ASPNET_WP.EXE orW3P.EXE).

ISAPI loads

HTTPRuntime

and passes the request to it. Thus, HTTP Pipelining has begun.

HTTPRuntime

uses

HttpApplicationFactory

to either create or reuse the

HTTPApplication

object.

HTTPRuntime

creates

HTTPContext

for the current request.

HTTPContext

internally maintains

HTTPRequest

and

HTTPResponse

. 

HTTPRuntime

also maps the

HTTPContext

to the

HTTPApplication

which handles the application level events.

HTTPApplication

runs the

HTTPModules

for the page requests.

HTTPApplication

creates

HTTPHandler

for the page request. This is the last stage of

HTTPipelining

.

HTTPHandlers

are responsible to process request and generate corresponding response messages.
Once the request leaves the

HTTPPipeline

, page level events begin.
PageEvents are as follows:

PreInit

,

Init

,

InitComplete

,

PreLoad

,

Load

,

Control

events (Postback events), Load Complete,

PreRender

,

SaveStateComplete

,

Render

and

Unload

.

HTTPHandler

generates the response with the above events and sends back to the IIS which in turn sends the response to the client browser.

Events in the Life Cycle of Page

PreInit: All the Pre and Post events are introduced as part of .NET Framework 2.0. As the name suggests, this event is fired before the

Init

method is fired. Most common functionalities implemented in this method include:

Check the

IsPostBack

property
Set the master page dynamically
Set the theme property of the page dynamically
Read or Set the profile property values
Re-create the dynamic controls
Init: This event is raised after all controls in the page are initialized and any skin settings have been applied. This event is used to read or initialize control properties. It can be used to register events for some controls for which
the events are not specified in the aspx page.

Ex:

OnClick

event of the Button can be registered in the

Init

rather than specifying in the

OnClick

property of the Button in the aspx page.

InitComplete: Use this event for processing tasks that require all initialization to be complete.

PreLoad: Use this event if you need to perform processing on your page or control before the

Load

event. After the

Page

raises this event, it loads view state for itself and all controls, and then processes any postback data included with the

Request

instance.

Load: The

Page

calls the

OnLoad

event method on the

Page

, then recursively does the same for each child control, which does the same for each of its child controls until the page and all controls are
loaded. Use the

OnLoad

event method to set properties in controls and establish database connections.

Controlevents: Use these events to handle specific control events, such as a

Button

control's

Click

event or a

TextBox

control's

TextChanged

event.

LoadComplete: Use this event for tasks that require that all other controls on the page be loaded.

PreRender: This is the last event raised before the HTML code is generated for the page. The

PreRender

event also occurs for each control on the page. Use the event to make final changes to the contents of the page or its controls.

SaveStateComplete: Before this event occurs,

ViewState

has been saved for the page and for all controls. Any changes to the page or controls at this point will be ignored.

Use this event to perform tasks that require view state to be saved, but that do not make any changes to controls.

Render: This is the stage where the HTML code for the page is rendered. The

Page

object calls the

Render

method of each control at this stage. All ASP.NET Web server controls have a

Render

method that writes out the control's markup that is sent to the browser.

UnLoad: This event occurs for each control and then for the page. In controls, use this event to do final cleanup for specific controls, such as closing control-specific database connections.

For the page itself, use this event to do final cleanup work, such as closing open files and database connections, or finishing up logging or other request-specific tasks.