ASP.NET Page Life Cycle

In this article I’ll talk about the ASP.NET page life cycle. We will try to see what all events are important for an ASP.NET developer and what can be achieved in these events.

As an ASP.NET developer, it is essential to understand the ASP.NET application life cycle and Page life cycle. With the ease of development provided by Visual Studio, sometimes new programmers get started with writing ASP.NET pages without understanding the Application and Page life cycle.

From an end user's perspective, a request for a web page is made to the web server and web server will return the page to the user. So simple isn’t it J

However for a little bit technical users, we can also state that the web server will receive the request, perform some server side activities like reading from database, process data received and return the output back to the user.

As an ASP.NET developer, one need to understand how this request is being processed i.e. the Application life cycle and how the web page is being processed and getting served to the user i.e. the Page life cycle.

When an ASP.NET page runs, the page goes through a life cycle in which it performs a series of processing steps.

Steps like; 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 better and at the appropriate stage of life-cycle for the effect you intend.

Let’s see Page life cycle stages

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. (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.

As I was said earlier; it is important for an ASP.NET developer and what can be achieved in these events, similarly we also need to understand in which event or stage of life cycle what is available to us.

For example ViewState is available in which all events or stages of a life cycle – see below image.

We can find list of all events available in Page Lifecycle at Microsoft MSDN Library

When we use server controls on ASP.net Page, we do not care about the individual controls life cycle – Yes each individual server control has its own life cycle which is similar to the page lifecycle.

For example: each control’s Init and Load event occurs during the corresponding page events

Below image can show you the flow of page life cycle – for detail information click here (reference of Microsoft MSDN Library)