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.
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Stage
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Description
7
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Page request
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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.
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Start
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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.
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Initialization
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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.
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Load
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During load, if the current request is a
postback, control properties are loaded with information recovered from view
state and control state.
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Postback event handling
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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.)
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Rendering
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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 OutputStreamobject
of the page's Response property.
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Unload
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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
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 asPage_Load and Page_Init. 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 custom ASP.NET server
controls, you need to understand more about these events.
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Page Event
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Typical Use
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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.
·
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.
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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.
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Raised at the end of the page's
initialization stage. Only one operation takes place between the Initand 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.
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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.
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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.
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Control events
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Use these events to handle specific control
events, such as a Button control's Click event or aTextBox control's TextChanged event.
Note |
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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.
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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 callsEnsureChildControls 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.
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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.
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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.
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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, seeDeveloping
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.
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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.Writemethod, the page will throw an exception.
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