ASP.Net Page Life Cycle

When the page is requested by a user, ASP.NET determines whether the page needs to be parsed and compiled, or whether a cached version of the page can be sent in response without running the page. At each stage of the page life cycle, the page raises some events, which could be coded.

Following are the page life cycle events:

PreInit: PreInit is the first event in page life cycle and raised before the initialization stage begins. Use this event for the following: 

1. Check the IsPostBack property to determine whether this is the first time the page is being processed. 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.
2. The IsCallback and IsCrossPagePostBack properties have also been set at this time.
3. Create or re-create dynamic controls.
4. Set a master page & theme property dynamically.
5. Read or set profile property values.

This event can be handled by overloading the OnPreInit method or creating a Page_PreInit handler.

Master page doesn't have PreInit method because Master pages are derived from Control class as seen in below hierarchy:

Init: This event is raised after all controls have been initialized and any skin settings have been applied. Init event initializes the control property and the control tree is built. The Init event of individual controls occurs before the Init event of the page. Use this event to read or initialize control properties.

This event can be handled by overloading the OnInit method or creating a Page_Init handler.

Order:

1.       Master page controls Init event

2.       Content controls Init event.

3.       Master page Init event.

4.       Content page Init event.

 

InitComplete: This event is raised at the end of the page's initialization stage. InitComplete event allows tracking of view state. All the controls turn on view-state tracking.

View state tracking enables controls to persist any values that are programmatically added to the ViewState collection. Use this event to make changes to view state that you want to make sure are persisted after the next postback.

 

PreLoad: This event is raised after the page loads view state for itself and all controls & occurs before the post back data is loaded in the controls

This event can be handled by overloading the OnPreLoad method or creating a Page_PreLoad handler.

Master page doesn't have PreLoad method.

Load: The Load event is raised for the page first and then recursively does the same for each child control until the page and all controls are loaded.

This event can be handled by overloading the OnLoad method or creating a Page_Load handler.

Order:

1.       Content page Load event.

2.       Master page Load event.

3.       Master page controls Load event.

4.       Content page controls Load event.

 

Load Complete: This event is 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.

This event can be handled by overloading the OnLoadComplete method or creating a Page_LoadComplete handler.

Master page doesn't have OnLoadComplete method.

 

PreRender: This event is raised after the Page object has created all controls that are required in order to render the page, including child controls of composite controls. The Page object raises the PreRender event on the Page, and then recursively does the same for each child control. Use the event to make final changes to the contents of the page or its controls before the rendering stage begins.

In this event, Page ensures that all child controls are created. Page calls EnsureChildControls for all controls, including itself. Every control whose datasource/databind property is set calls for its databind method.

Order:

5.       Content page PreRender event.

6.       Master page PreRender event.

7.       Master page controls PreRender event.

8.       Content page controls PreRender event.

 

PreRenderComplete: This event is raised after each control's PreRender property is completed. This event ensures the completion of the pre-rendering phase.

SaveStateComplete: This event is 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 persist on the next postback.

Render: Every ASP.NET control has render method and the page instance calls this method to output the control’s markup, after this event any changes to the page or controls are ignored.

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 automatically incorporates rendering, so you do not need to explicitly render the control in code.

 

Unload: This event is raised for each control and then for the page. the UnLoad phase is the last phase of the page life cycle. 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.

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.

The following is the sequence in which events occur when a master page is merged with a content page:

1.       Content page PreInit event.

2.       Master page controls Init event.

3.       Content controls Init event.

4.       Master page Init event.

5.       Content page Init event.

6.       Content page PreLoad event

7.       Content page Load event.

8.       Master page Load event.

9.       Master page controls Load event.

10.   Content page controls Load event.

11.   Content Page LoadComplete event.

12.   Content page PreRender event.

13.   Master page PreRender event.

14.   Master page controls PreRender event.

15.   Content page controls PreRender event.

16.   Master page controls Unload event.

17.   Content page controls Unload event.

18.   Master page Unload event.

19.   Content page Unload event.