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. Additionally, if you develop custom controls, you must be familiar with the page life cycle in order to correctly initialize controls, populate control properties with view state data, and run any control behavior logic. (The life cycle of a control is based on the page life cycle, but the page raises more events for a control than are available for an ASP.NET page alone.)

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. For more information, see ASP.NET Application Life Cycle Overview.

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 step, 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. Additionally, during the start step, the page's UICulture property is set.
Page initialization	During page initialization, controls on the page are available and each control's UniqueID property is set. Any themes are also applied to the page. 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.
Validation	During validation, the Validate method of all validator controls is called, which sets the IsValid property of individual validator controls and of the page.
Postback event handling	If the request is a postback, any event handlers are called.
Rendering	During rendering, view state is saved to the page and then the page calls on each control to contribute its rendered output to the OutputStream of the page's Response property.
Unload	Unload is called when 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 any cleanup is performed.

Life Cycle Events

Within each stage of the life cycle of a page, the page raises events that you can handle to run your own code. For control events, you bind the event handler to the event, either declaratively using attributes such as onclick, or in code.

Pages also support auto 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 (or if it is not defined, since by default it is true), page events are automatically bound to methods that use the naming convention of Page_event, such as Page_Load and Page_Init. For more information on auto event wire-up, see ASP.NET Web Server Control Event Model.

The following table lists the page life-cycle events that you will use most frequently. There are more events than those listed. However, they are not used for most page processing scenarios. Instead, they are primarily used by server controls on the ASP.NET Web page to initialize and render themselves. If you want to write your own ASP.NET server controls, you need to understand more about these stages. For information creating custom control, see Developing Custom ASP.NET Server Controls.

Page Event

Typical Use

Page_PreInit

Use the IsPostBack property to determine whether this is the first time the page is being processed.
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 stage.

Page_Init

Read or initialize control properties.

Page_Load

Read and update control properties.

Control events

Perform your application-specific processing:

If the page contains validator controls, check the IsValid property of the page and of individual validation controls before performing any processing.
Handle the specific event, such as a Button control's Click event.

Page_PreRender

Make final changes to the contents of the page.

Page_Unload

Perform final cleanup work, which might include:

Closing open files and database connections.

Finishing 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.

Additional Page Life Cycle Considerations

Note the following additional information about the page life cycle:

Individual ASP.NET server controls have their own life cycle that is similar to the page life cycle. For example, a control's Init and Load methods are called during the corresponding page events. If you have a control on the page, the control's Init method will be called first followed by the page's Init method. However, the page's Load method will be called before the control's Load method.
You can customize the appearance or content of a control by handling the events for the control. For example, all controls raise Init, Load, and Unload events, although it is not common for page developers to handle those events. It is more common to handle the events that are specific to the control, such as the Click event for the Button control and the SelectedIndexChanged event for the ListBox control. Under some circumstances, you might also handle a control's DataBinding or DataBound events. For more information, see the class reference topics for individual controls and Developing Custom ASP.NET Server Controls.
In addition to handling events raised by the page, you can override methods from the page's base class. For example, you can override the page's InitializeCulture method to dynamically set culture information. Note that when creating an event handler using the Page_event syntax, the base implementation is implicitly called and therefore you do not need to call it in your method. For example, the base page class's OnLoad method is always called, whether you create a Page_Load method or not. However, if you override the page OnLoad method with the override keyword (Overrides in Visual Basic), you must explicitly call the base method. For example, if you override the OnLoad method on the page, you must call base.Load (MyBase.Load in Visual Basic) in order for the base implementation to be run.

General Page Life Cycle Stages

Life Cycle Events

Additional Page Life Cycle Considerations

See Also

Reference

Concepts

Other Resources