Understanding ASP.NET Page Life Cycle

PreInit

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.

• 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. 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.

Note
In 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.

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