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Understanding ASP.NET Page Life Cycle

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	codeling 1595 - 6639
	@2017-02-08 16:16:09

When an ASP.NET page runs, the page goes through a life cycle in which it performs a series of processing steps. These include initialization, instantiating controls, restoring and maintaining state, running event handler code, and rendering. It is important for you to understand the page life cycle so that you can write code at the appropriate life-cycle stage for the effect you intend.

@2018-01-12 09:04:17

General Page Life-Cycle Stages

In general terms, the page goes through the stages outlined in the following table. In addition to the page life-cycle stages, there are application stages that occur before and after a request but are not specific to a page.

Some parts of the life cycle occur only when a page is processed as a postback. For postbacks, the page life cycle is the same during a partial-page postback (as when you use an UpdatePanel control) as it is during a full-page postback.

Stage	Description
Page request	The page request occurs before the page life cycle begins. When the page is requested by a user, ASP.NET determines whether the page needs to be parsed and compiled (therefore beginning the life of a page), or whether a cached version of the page can be sent in response without running the page.
Start	In the start stage, page properties such as Request and Response are set. At this stage, the page also determines whether the request is a postback or a new request and sets the IsPostBack property. The page also sets the UICulture property.
Initialization	During page initialization, controls on the page are available and each control's UniqueID property is set. A master page and themes are also applied to the page if applicable. If the current request is a postback, the postback data has not yet been loaded and control property values have not been restored to the values from view state.
Load	During load stage, if the current request is a postback, control properties are loaded with information recovered from view state and control state.
Postback event handling	If the request is a postback, control event handlers are called. After that, the Validate method of all validator controls is called, which sets the IsValid property of individual validator controls and of the page. (There is an exception to this sequence: the handler for the event that caused validation is called after validation.)
Rendering	Before rendering, view state is saved for the page and all controls. During the rendering stage, the page calls the Render method for each control, providing a text writer that writes its output to the OutputStream object of the page's Response property.
Unload	The Unload event is raised after the page has been fully rendered, sent to the client, and is ready to be discarded. At this point, page properties such as Response and Request are unloaded and cleanup is performed.

@2018-01-12 09:42:17

ASP.NET Application Life Cycle

Each time a request arrives at a Web server for an ASP.NET Web page, the first thing the Web server does is hand off the request to the ASP.NET engine. The ASP.NET engine then takes the request through a pipeline composed of numerous stages, which includes verifying file access rights for the ASP.NET Web page, resurrecting the user's session state, and so on. At the end of the pipeline, a class corresponding to the requested ASP.NET Web page is instantiated and the ProcessRequest() method is invoked.

Click here for larger image.

IIS 6.0 & ASP.NET Pipelines

In IIS 6.0 and previous releases, ASP.NET was implemented as an IIS ISAPI extension.+

In these earlier releases, IIS processed a request to an ASP.NET content type and then forwarded that request to the ASP.NET ISAPI DLL, which hosted the ASP.NET request pipeline and page framework. Requests to non-ASP.NET content, such as ASP pages or static files, were processed by IIS or other ISAPI extensions and were not visible to ASP.NET.+

The major limitation of this model was that services provided by ASP.NET modules and custom ASP.NET application code were not available to non-ASP.NET requests. In addition, ASP.NET modules were unable to affect certain parts of the IIS request processing that occurred before and after the ASP.NET execution path.+

IIS 7 and Above Integrated Mode

The main difference in processing stages between IIS 7.0 and IIS 6.0 is in how ASP.NET is integrated with the IIS server. In IIS 6.0, there are two request processing pipelines. One pipeline is for native-code ISAPI filters and extension components. The other pipeline is for managed-code application components such as ASP.NET. In IIS 7.0, the ASP.NET runtime is integrated with the Web server so that there is one unified request processing pipeline for all requests.

IIS processes requests that arrive for any content type, with both native IIS modules and ASP.NET modules providing request processing in all stages. This enables services that are provided by ASP.NET modules, such as Forms authentication or output cache, to be used for requests to ASP pages, PHP pages, static files, and so on.+

The ability to plug in directly into the server pipeline allows ASP.NET modules to replace, run before, or run after any IIS functionality. This enables, for example, a custom ASP.NET Basic authentication module that is written to use the Membership service and SQL Server user database to replace the built-in IIS Basic authentication feature that works only with Windows accounts.+

In addition, the expanded ASP.NET APIs use direct integration to enable more request-processing tasks. For example, ASP.NET modules can modify request headers before other components process the request, by inserting an Accept-Language header before ASP applications execute, which forces localized content to be sent back to the client based on user preference.

Because of the runtime integration, IIS and ASP.NET can use the same configuration to enable and order server modules, and to configure handler mappings. Other unified functionality includes tracing, custom errors, and output caching.

ASP.NET Page Life Cycle

This life cycle of the ASP.NET page starts with a call to the ProcessRequest() method. This method begins by initializing the page's control hierarchy. Next, the page and its server controls proceed lock-step through various phases that are essential to executing an ASP.NET Web page. These steps include managing view state, handling postback events, and rendering the page's HTML markup. Figure 2 provides a graphical representation of the ASP.NET page life cycle. The life cycle ends by handing off the Web page's HTML markup to the Web server, which sends it back to the client that requested the page.

ms972976.viewstate_fig02(en-us,MSDN.10).gif

@2018-01-12 10:01:42

HTML forms use either GET or POST to send data to the server. The method attribute of the form element gives the HTTP method:

The default method is GET. If the form uses GET, the form data is encoded in the URI as a query string. If the form uses POST, the form data is placed in the request body.

ASP.Net sets the method to POST for the form. The following ASP.NET page will set the HTTP method to post when method="" attribute is not including the in the form declaration:

<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title>Ny test</title>
    <link rel="stylesheet" type="text/css" href="StyleSheet.css" />
</head>
<body>

    <form action="Default.aspx" runat="server">

    Name: <input type="text" id="navn" runat="server"/>
    <input type="submit" id="submit" value="Submit" runat="server" />
    <input type="reset" />
    <br />

    </form>
</body>
</html>

The HTML code above results in the following output in the URL after the submit button is clicked:

http://localhost/demowebsite/Default.aspx

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