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
General Page Life-cycle Stages
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.
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: Before rendering, view state is saved for the page and all controls. During the rendering phase, the page calls the Render method for each control, providing a text writer that writes its output to the OutputStream of the page's Response property
Unload: Unload is called 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 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
PreInit: The PreInit event is an event early in the page life cycle that you can access. After the PreInit event, personalization information and the page theme, if any, is loaded.
Use this event for the following:
- Check 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 event.
Init:Raised after all controls have been initialized and any skin settings have been applied. Use this event to read or initialize control properties.
InitComplete: Raised by the Page object. Use this event for processing tasks that require all initialization be complete.
PreLoad: Use this event if you need to perform processing on your page or control before the Load event.
Before the Page instance raises this event, it loads view state for itself and all controls, and then processes any postback data included with the Request instance.
Load: The Page calls the OnLoad event method on the Page, then recursively does the same for each child control, which does the same for each of its child controls until the page and all controls are loaded.
Use the OnLoad event method to set properties in controls and establish database connections.
LoadComplete: Use this event for tasks that require that all other controls on the page be loaded.
PreRender: The PreRender event occurs for each control on the page. Use the event to make final changes to the contents of the page or its controls.
SaveStateComplete:Before this event occurs, ViewState has been saved for the page and for all controls. Any changes to the page or controls at this point will be ignored.
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 that is sent 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.
A user control (an .ascx file) automatically incorporates rendering, so you do not need to explicitly render the control in code.
Unload: This event occurs 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.
Init:Raised after all controls have been initialized and any skin settings have been applied. Use this event to read or initialize control properties.
InitComplete: Raised by the Page object. Use this event for processing tasks that require all initialization be complete.
PreLoad: Use this event if you need to perform processing on your page or control before the Load event.
Before the Page instance raises this event, it loads view state for itself and all controls, and then processes any postback data included with the Request instance.
Load: The Page calls the OnLoad event method on the Page, then recursively does the same for each child control, which does the same for each of its child controls until the page and all controls are loaded.
Use the OnLoad event method to set properties in controls and establish database connections.
LoadComplete: Use this event for tasks that require that all other controls on the page be loaded.
PreRender: The PreRender event occurs for each control on the page. Use the event to make final changes to the contents of the page or its controls.
SaveStateComplete:Before this event occurs, ViewState has been saved for the page and for all controls. Any changes to the page or controls at this point will be ignored.
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 that is sent 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.
A user control (an .ascx file) automatically incorporates rendering, so you do not need to explicitly render the control in code.
Unload: This event occurs 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.
