In this article, we are going to discuss the different methods and order they are executed during the load of an .aspx web page.
When a user first requests an .aspx page on your server, the server sends it to the HTTP Pipeline. The HTTP Pipeline handles all processes involved in converting all of the application code into HTML to be interpreted by the browser. The first class initiated is called HttpRuntime. This class finds a free HttpApplication object to start processing the request. The HttpApplication object then runs the appropriate handler assigned in the web.config and machine.config files for the requested extension.
The extension .aspx can be handled by the HandlerClass or HandlerFactory class. The HttpApplication objects starts the IHttpHandler interface which begins processing the application code by calling the processRequest() method.
The processRequest() method then calls the FrameworkInitialize() method which begins building the control trees for the requested page. Now the processRequest() method cycles through the page's life cycle in the order listed below.
(1) PreInit The entry point of the page life cycle is the pre-initialization phase called “PreInit”. This is the only event where programmatic access to master pages and themes is allowed. You can dynamically set the values of master pages and themes in this event. You can also dynamically create controls in this event.
The extension .aspx can be handled by the HandlerClass or HandlerFactory class. The HttpApplication objects starts the IHttpHandler interface which begins processing the application code by calling the processRequest() method.
The processRequest() method then calls the FrameworkInitialize() method which begins building the control trees for the requested page. Now the processRequest() method cycles through the page's life cycle in the order listed below.
(1) PreInit The entry point of the page life cycle is the pre-initialization phase called “PreInit”. This is the only event where programmatic access to master pages and themes is allowed. You can dynamically set the values of master pages and themes in this event. You can also dynamically create controls in this event.
(2)Init This event fires after each control has been initialized, each control's UniqueID is set and any skin settings have been applied. You can use this event to change initialization values for controls. The “Init” event is fired first for the most bottom control in the hierarchy, and then fired up the hierarchy until it is fired for the page itself.
(3)InitComplete Raised once all initializations of the page and its controls have been completed. Till now the view state values are not yet loaded, hence you can use this event to make changes to view state that you want to make sure are persisted after the next postback.
(4)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
(1)Loads ViewState : ViewState data are loaded to controls
(2)Loads Postback data : postback data are now handed to the page controls During this phase of the page creation, form data that was posted to the server (termed postback data in ASP.NET) is processed against each control that requires it. Hence, the page fires the LoadPostData event and parses through the page to find each control and updates the control state with the correct postback data. ASP.NET updates the correct control by matching the control's unique ID with the name/value pair in the NameValueCollection. This is one reason that ASP.NET requires unique IDs for each control on any given page.
(5)Load The important thing to note about this event is the fact that by now, the page has been restored to its previous state in case of postbacks. Code inside the page load event typically checks for PostBack and then sets control properties appropriately. This method is typically used for most code, since this is the first place in the page lifecycle that all values are restored. Most code checks the value of IsPostBack to avoid unnecessarily resetting state. You may also wish to call Validate and check the value of IsValid in this method. You can also create dynamic controls in this method.
(6)Control (PostBack) event(s)ASP.NET now calls any events on the page or its controls that caused the PostBack to occur. This might be a button’s click event or a dropdown's selectedindexchange event, for example.These are the events, the code for which is written in your code-behind class(.cs file).
(7)LoadComplete This event signals the end of Load.
(8)PreRender Allows final changes to the page or its control. This event takes place after all regular PostBack events have taken place. This event takes place before saving ViewState, so any changes made here are saved.For example : After this event, you cannot change any property of a button or change any viewstate value. Because, after this event, SaveStateComplete and Render events are called.
(9)SaveStateComplete Prior to this event the view state for the page and its controls is set. Any changes to the page’s controls at this point or beyond are ignored.
(10)Render This is a method of the page object and its controls (and not an event). At this point, ASP.NET calls this method on each of the page’s controls to get its output. The Render method generates the client-side HTML, Dynamic Hypertext Markup Language (DHTML), and script that are necessary to properly display a control at the browser.
(11)UnLoad This event is used for cleanup code. After the page's HTML is rendered, the objects are disposed of. During this event, you should destroy any objects or references you have created in building the page. At this point, all processing has occurred and it is safe to dispose of any remaining objects, including the Page object. Cleanup can be performed on-
(a)Instances of classes i.e. objects
(b)Closing opened files
(c)Closing database connections. 
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