If we create a web Page, which consists of one or more Web Controls that are configured to use AutoPostBack (every Web controls will have their own AutoPostBack property), the ASP.NET adds a special JavaScipt function to the rendered HTML Page. This function is named _doPostBack() . When Called, it triggers a PostBack, sending data back to the web Server.
ASP.NET also adds two additional hidden input fields that are used to pass information back to the server. This information consists of ID of the Control that raised the event and any additional information if needed. These fields will empty initially as shown below,
- <input type="hidden" name="__EVENTTARGET" id="__EVENTTARGET" value="" />
- <input type="hidden" name="__EVENTARGUMENT" id="__EVENTARGUMENT" value="" />
The following actions will be taken place when a user changes a control that has the AutoPostBack property set to true:
- On the client side, the JavaScript _doPostBack function is invoked, and the page is resubmitted to the server.
- ASP.NET re-creates the Page object using the .aspx file.
- ASP.NET retrieves state information from the hidden view state field and updates the controls accordingly.
- The Page.Load event is fired.
- The appropriate change event is fired for the control. (If more than one control has been changed, the order of change events is undetermined.)
- The Page.PreRender event fires, and the page is rendered (transformed from a set of objects to an HTML page).
- Finally, the Page.Unload event is fired.
- The new page is sent to the client.
