ASP.NET Page Life-Cycle Events

Since the beginning days i was working on with ASP.NET 1.1, quite familiar with the life-cycle events within the ASP.NET 1.1.

But still it will be difficult to remember everything, when we are asked unexpectedly, since my memory management is poorer. :-).

Recently in one interview, while doing the management round interview discussion, i have been asked to explain ASP.NET Page life cycle, since i didnt expected the question in managarial round and i was quite surprised, i have tried to explain events only i remeber. like init(), load(), pre_render(), render() etc…

My interviewer asked me what is PreLoad(),  at first i was confused, would there been any such event? so i said i am not sure about it.

Later after coming out of interview i remembered ohh!!, There is a “Pre_Load” event in ASP.NET 2.0 and above versions page life cycle to handle the post back requests,  is raised after all postback data processing, just before the Load() event.  Since i am mostly familiar with ASP.NET 1.1,  i got to express to my interviewer that i am not sure about it. Mistakes happen accidently. I was aware of such event and i couldn’t fetch it in my mind on right time.

This event occurs just before Load(), Raised after the page loads view state for itself and all controls, and after it processes postback data that is included with the post back Request instance.

Anyway i started the topic. I will just give some brief description about the life cycle events, by refering to MSDN Library.

What is a life Cycle?

A life cycle, is the way a program or application handles/performs a series of processing steps. This could be a certain number of steps or methods or events called in Sequencial Order, based on the process handles the request.

General/Common Page Life-Cycle Stages or Events

These events are most commonly occured in an ASP.NET Page life cycle.  I will lend a tabular data from MSDN Library to explain this.

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, 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.
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.

Life-Cycle Events

Life Cycle Events are events which are occured based on the scenario and usage of the Page. Suppose if we consider “Pre_Load” event, will not occur when the page is loaded for the first time. Pre_Load event occurs when the page is post backed.

Lets see all the events in details, and you will get much more idea.

ASP.NET Pages also support automatic event wire-up, meaning that ASP.NET looks for methods with particular names and automatically runs those methods when certain events are raised. If the AutoEventWireup attribute of the @ Page directive is set to true, page events are automatically bound to methods that use the naming convention of Page_event, such as Page_Load and Page_Init.

The following table lists the page life-cycle events that you will use most frequently.

Page Event	Typical Use
PreInit	Raised after the start stage is complete and before the initialization stage begins.Use this event for the following: Check the IsPostBack property to determine whether this is the first time the page is being processed. The IsCallback and IsCrossPagePostBack properties have also been set at this time. Create or re-create dynamic controls. Set a master page dynamically. Set the Theme property dynamically. Read or set profile property values. Note If the request is a postback, the values of the controls have not yet been restored from view state. If you set a control property at this stage, its value might be overwritten in the next event.
Init	Raised after all controls have been initialized and any skin settings have been applied. The Init event of individual controls occurs before the Init event of the page.Use this event to read or initialize control properties.
InitComplete	Raised at the end of the page’s initialization stage. Only one operation takes place between the Init and InitComplete events: tracking of view state changes is turned on. View state tracking enables controls to persist any values that are programmatically added to the ViewState collection. Until view state tracking is turned on, any values added to view state are lost across postbacks. Controls typically turn on view state tracking immediately after they raise their Init event.Use this event to make changes to view state that you want to make sure are persisted after the next postback.
PreLoad	Raised after the page loads view state for itself and all controls, and after it processes postback data that is included with the Request instance.
Load	The Page object calls the OnLoad method on the Page object, and then recursively does the same for each child control until the page and all controls are loaded. The Load event of individual controls occurs after the Load event of the page.Use the OnLoad event method to set properties in controls and to establish database connections.
Control events	Use these events to handle specific control events, such as a Button control’s Click event or a TextBox control’s TextChanged event. Note In a postback request, if the page contains validator controls, check the IsValid property of the Page and of individual validation controls before performing any processing.
LoadComplete	Raised at the end of the event-handling stage.Use this event for tasks that require that all other controls on the page be loaded.
PreRender	Raised after the Page object has created all controls that are required in order to render the page, including child controls of composite controls. (To do this, the Page object calls EnsureChildControls for each control and for the page.)The Page object raises the PreRender event on the Page object, and then recursively does the same for each child control. The PreRender event of individual controls occurs after the PreRender event of the page.Use the event to make final changes to the contents of the page or its controls before the rendering stage begins.
PreRenderComplete	Raised after each data bound control whose DataSourceID property is set calls its DataBind method. For more information, see Data Binding Events for Data-Bound Controls later in this topic.
SaveStateComplete	Raised after view state and control state have been saved for the page and for all controls. Any changes to the page or controls at this point affect rendering, but the changes will not be retrieved on the next postback.
Render	This is not an event; instead, at this stage of processing, the Page object calls this method on each control. All ASP.NET Web server controls have a Render method that writes out the control’s markup to send to the browser.If you create a custom control, you typically override this method to output the control’s markup. However, if your custom control incorporates only standard ASP.NET Web server controls and no custom markup, you do not need to override the Render method. For more information, see Developing Custom ASP.NET Server Controls.A user control (an .ascx file) automatically incorporates rendering, so you do not need to explicitly render the control in code.
Unload	Raised 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. Note During the unload stage, the page and its controls have been rendered, so you cannot make further changes to the response stream. If you attempt to call a method such as the Response.Write method, the page will throw an exception.

READ MORE HERE…………

This is just an information sharing blog which i felt i should share it. The actual source of this content is MSDN Library link about “ASP.NET Page Life Cycle Overview”

One more excellent link i would like to share is “Shivprasad Koirala – Microsoft MVP’s – Article in CodeProject.com”

Cheat Sheet for ASP.NET Page Life Cycle   (PDF)

Cheat Sheet for ASP.NET Full Event Life Cycle  (PDF)

People learn from mistakes and i learned from the mistake i did and i believe i will remember these always. A true acknowledgable interview i had. Thanks to my interviewer(it’s not right mentioning, name of my interviewer) , Shivprasad Koirala and Microsoft MSDN library.