A page in an ASP.NET application consists of several server controls. These are the fundamental building blocks of an ASP.NET application. The Life cycle of an ASP.NET page, depends on whether the page is requested for the first time or it is a postback. Postback is a process by which a page can request for itself.
When the Page is requested for the first time
The Life Cycle of a page when requested for the first time:
Initializing: During this phase, the server creates an instance of the server control
Loading: During this phase, the instance of the control is loaded onto the page object in which it is defined.
PreRendering: During this phase, the control is updated with the changes made to it. This prepares the control for rendering.
Saving: During this phase, the state information of the control is saved. For example, if a value is set for the control during the Load event, it is embedded in the HTML tag that will be returned to the browser.
Rendering: During this phase, the server creates the corresponding HTML tag for the control.
Disposing: During this phase, all cleanup tasks, such as closing files and database connections opened by the control are performed.
Unloading: During this phase, all cleanup tasks, such as destroying the instances of server control are performed. This is the final event in the life cycle of a server control
Life cycle when the page processed during a postback event
The processing sequence in which a page is processed during a postback event is:
Initializing: During this phase, the server creates an instance of the server control
Loading view state: During this phase, the view state of the control posted by the client is reloaded into the new instance of the control.
Loading: During this phase, the instance of the control is loaded onto the page object in which it is defined.
Loading the postback data: During this phase, the server searches any data corresponding to the control that is loaded in the data posted by the client.
PreRendering: During this phase, the control is updated with the changes made to it. This prepares the control for rendering.
Saving state: During this phase, the change in the state of control between the current request and the previous request of the page is saved. For each change, the corresponding event is raised. For example, if the text of a textbox is changed, the new text is saved and a text_change event is raised.
Rendering: During this phase, the server creates the corresponding HTML tag for the control.
Disposing: During this phase, all cleanup tasks, such as closing files and database connections opened by the control are performed.
Unloading: During this phase, all cleanup tasks, such as destroying the instances of server control are performed. This is the final event in the life cycle of a server control
The events associated with the relevant page cycle phases are:
- Page Initialization: Page_Init
- View State Loading:LoadViewState
- Postback data processing: LoadPostData
- Page Loading: Page_Load
- PostBack Change Notification: RaisePostDataChangedEvent
- PostBack Event Handling: RaisePostBackEvent
- Page Pre Rendering Phase: Page_PreRender
- View State Saving: SaveViewState
- Page Rendering: Page_Render
- Page Unloading: Page_UnLoad
General Page Life Cycle Stages: (As per MSDN)
| 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. |
| 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 performe |
Life-Cycle Events(As per MSDN) :
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 as Page_Load and Page_Init.
| Page Event | Typical Use |
|---|---|
| PreInit | Raised after the start stage is complete and before the initialization stage begins. Use this event for the following:
|
| Init | 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. |
| InitComplete | Raised at the end of the page's initialization stage. Only one operation takes place between the Init and 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. |
| PreLoad | 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. |
| Load | 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. |
| Control events | Use these events to handle specific control events, such as a Button control's Click event or a TextBox control's TextChanged event. NoteIn a postback request, if the page contains validator controls, check the IsValid property of the Page and of individual validation controls before performing any processing. |
| LoadComplete | 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. |
| PreRender | 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 calls EnsureChildControls 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. |
| PreRenderComplete | 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. |
| SaveStateComplete | 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. |
| 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 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, see Developing 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. |
| Unload | 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. NoteDuring 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. |
The following figure illustrates how the server controls on an ASP.NET page is processed by the server:
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