Technology Excellence  |  General Industry Practice  | Healthcare IT Solutions | Profile | Route Map | Site Map | Contact Us | Search Home Healthcare Apps Solutions Projects ASP.NET Application Life Cycle Overview Within ASP.NET, several processing steps must occur for an ASP.NET application to be initialized and process requests. Additionally, ASP.NET is only one piece of the Web server architecture that services requests made by browsers. It is important for you to understand the application life cycle so that you can write code at the appropriate life cycle stage for the effect you intend. The following Contents describes the stages of the ASP.NET application life cycle. Please fill up the form below and we will submit a proposal for your project. Alternatively, you can send email to contact@optionm.net with the project requirements. Contents ASP.NET Application Life Cycle for IIS 5.0 and 6.0 ASP.NET Application Life Cycle for IIS 7.0 ASP.NET Page Life Cycle ASP.NET Application Life Cycle for IIS 5.0 and 6.0 This topic outlines the life cycle of ASP.NET applications, listing important life-cycle events and describing how code that you write can fit into the application life cycle. The information in this topic applies to IIS 5.0 and IIS 6.0. Application Life Cycle in General: The following sections describes the stages of the ASP.NET application life cycle. Application resource from the User requests a Web server. The life cycle of an ASP.NET application starts with a request sent by a browser to the Web server (for ASP.NET applications, typically IIS). ASP.NET is an ISAPI extension under the Web server. When a Web server receives a request, it examines the file-name extension of the requested file, determines which ISAPI extension should handle the request, and then passes the request to the appropriate ISAPI extension. ASP.NET handles file name extensions that have been mapped to it, such as .aspx, .ascx, .ashx, and .asmx. Note: If a file name extension has not been mapped to ASP.NET, ASP.NET will not receive the request. This is important to understand for applications that use ASP.NET authentication. For example, because .htm files are typically not mapped to ASP.NET, ASP.NET will not perform authentication or authorization checks on requests for .htm files. Therefore, even if a file contains only static content, if you want ASP.NET to check authentication, create the file using a file name extension mapped to ASP.NET, such as .aspx. Note: If you create a custom handler to service a particular file name extension, you must map the extension to ASP.NET in IIS and also register the handler in your application's Web.config file. Application resource from the User requests an Web server. The life cycle of an ASP.NET application starts with a request sent by a browser to the Web server (for ASP.NET applications, typically IIS). ASP. requested file, determines which ISAPI extension should handle the request, and then passes the request to the appropriate ISAPI extension. ASP.NET handles file name extensions that have been mapped to it, such as .aspx, .ascx, .ashx, and .asmx. NET is an ISAPI extension under the Web server. When a Web server receives a request, it examines the file-name extension of the ASP.NET receives the first request for the application. When ASP.NET receives the first request for any resource in an application, a class named Application Manager creates an application domain. Application domains provide isolation between applications for global variables and allow each application to be unloaded separately. Within an application domain, an instance of the class named Hosting Environment is created, which provides access to information about the application such as the name of the folder where the application is stored. The following diagram illustrates this relationship: ASP.NET also compiles the top-level items in the application if required, including application code in the App_Code folder. For more information, see "Compilation Life Cycle" later in this topic. ASP.NET core objects are created for each request. After the application domain has been created and the HostingEnvironment object instantiated, ASP.NET creates and initializes core objects such as HttpContext, HttpRequest, and HttpResponse. The HttpContext class contains objects that are specific to the current application request, such as the HttpRequest and HttpResponse objects. The HttpRequest object contains information about the current request, including cookies and browser information. The HttpResponse object contains the response that is sent to the client, including all rendered output and cookies. An HttpApplication object is assigned to the request Note: After all core application objects have been initialized, the application is started by creating an instance of the HttpApplication class. If the application has a Global.asax file, ASP.NET instead creates an instance of the Global.asax class that is derived from the HttpApplication class and uses the derived class to represent the application. Note: The first time an ASP.NET page or process is requested in an application, a new instance of HttpApplication is created. However, to maximize performance, HttpApplication instances might be reused for multiple requests. When an instance of HttpApplication is created, any configured modules are also created. For instance, if the application is configured to do so, ASP.NET creates a SessionStateModule module. After all configured modules are created, the HttpApplication class's Init method is called. The following diagram illustrates the relationship The request is processed by the HttpApplication pipeline. The following events are executed by the HttpApplication class while the request is processed. The events are of particular interest to developers who want to extend the HttpApplication class. 10. Validate the request, which examines the information sent by the browser and determines whether it contains potentially malicious markup. Perform URL mapping, if any URLs have been configured in the UrlMappingsSection section of the Web.config file. 1. Raise the BeginRequest event. 2. Raise the AuthenticateRequest event. 3. Raise the PostAuthenticateRequest event. 4. Raise the AuthorizeRequest event. 5. Raise the PostAuthorizeRequest event. 6. Raise the ResolveRequestCache event. 7. Raise the PostResolveRequestCache event. 8. Based on the file name extension of the requested resource (mapped in the application's configuration file), select a class that implements IHttpHandler to process the request. If the request is for an object (page) derived from the Page class and the page needs to be compiled, ASP.NET compiles the page before creating an instance of it. 9. Raise the PostMapRequestHandler event. 10. Raise the AcquireRequestState event. 11. Raise the PostAcquireRequestState event. 12. Raise the PreRequestHandlerExecute event. 13. Call the ProcessRequest method (or the asynchronous version IHttpAsyncHandler..::.BeginProcessRequest) of the appropriate IHttpHandler class for the request. For example, if the request is for a page, the current page instance handles the request. 14. Raise the PostRequestHandlerExecute event. 15. Raise the ReleaseRequestState event. 16. Raise the PostReleaseRequestState event. 17. Perform response filtering if the Filter property is defined. 18. Raise the UpdateRequestCache event. 19. Raise the PostUpdateRequestCache event. 20. Raise the EndRequest event. 21. Raise the PreSendRequestHeaders event. 22. Raise the PreSendRequestContent event. Life Cycle Events and the Global.asax file During the application life cycle, the application raises events that you can handle and calls particular methods that you can override. To handle application events or methods, you can create a file named Global.asax in the root directory of your application. If you create a Global.asax file, ASP.NET compiles it into a class derived from the HttpApplication class, and then uses the derived class to represent the application. An instance of HttpApplication processes only one request at a time. This simplifies application event handling because you do not need to lock non-static members in the application class when you access them. This also allows you to store request-specific data in non-static members of the application class. For example, you can define a property in the Global.asax file and assign it a request-specific value. ASP.NET automatically binds application events to handlers in the Global.asax file using the naming convention Application_event, such as Application_BeginRequest. This is similar to the way that ASP.NET page methods are automatically bound to events, such as the page's Page_Load event.The Application_Start and Application_End methods are special methods that do not represent HttpApplication events. ASP.NET calls them once for the lifetime of the application domain, not for each HttpApplication instance.The following Section lists some of the events and methods that are used during the application life cycle. There are many more events than those listed, but they are not commonly used. Application_Start Called when the first resource (such as a page) in an ASP.NET application is requested. The Application_Start method is called only one time during the life cycle of an application. You can use this method to perform startup tasks such as loading data into the cache and initializing static values. You should set only static data during application start. Do not set any instance data because it will be available only to the first instance of the HttpApplication class that is created. Application_ event Raised at the appropriate time in the application life cycle, as listed in the application life cycle table earlier in this topic.Application_Error can be raised at any phase in the application life cycle. Application_EndRequest is the only event that is guaranteed to be raised in every request, because a request can be short-circuited. For example, if two modules handle the Application_BeginRequest event and the first one throws an exception, the Application_BeginRequest event will not be called for the second module. However, the Application_EndRequest method is always called to allow the application to clean up resources. Init Called once for every instance of the HttpApplication class after all modules have been created. Dispose Called before the application instance is destroyed. You can use this method to manually release any unmanaged resources. Application_End Called once per lifetime of the application before the application is unloaded. Compilation Life Cycle When the first request is made to an application, ASP.NET compiles application items in a specific order. The first items to be compiled are referred to as the top-level items. After the first request, the top-level items are recompiled only if a dependency changes. The following sections describes the order in which ASP.NET top-level items are compiled. App_GlobalResources The application's global resources are compiled and a resource assembly is built. Any assemblies in the application's Bin folder are linked to the resource assembly. App_WebResources Proxy types for Web services are created and compiled. The resulting Web references assembly is linked to the resource assembly if it exists. Profile properties defined in the Web.config file If profile properties are defined in the application's Web.config file, an assembly is generated that contains a profile object. App_Code Source code files are built and one or more assemblies are created. All code assemblies and the profile assembly are linked to the resources and Web references assemblies if any. Global.asax The application object is compiled and linked to all of the previously generated assemblies. After the application's top level items have been compiled, ASP.NET compiles folders, pages, and other items as needed. The following table describes the order in which ASP.NET folders and items are compiled. Location Description App_LocalResources If the folder containing the requested item contains an App_LocalResources folder, the contents of the local resources folder are compiled and linked to the global resources assembly. Individual Web pages (.aspx files), user controls (.ascx files), HTTP handlers (.ashx files), and HTTP modules (.asmx files) Compiled as needed and linked to the local resources assembly and the top-level assemblies. Themes, master pages, other source files Skin files for individual themes, master pages, and other source code files referenced by pages are compiled when the referencing page is compiled. Compiled assemblies are cached on the server and reused on subsequent requests and are preserved across application restarts as long as the source code is unchanged. Because the application is compiled on the first request, the initial request to an application can take significantly longer than subsequent requests. You can precompile your application to reduce the time required for the first request. Application Restarts Modifying the source code of your Web application will cause ASP.NET to recompile source files into assemblies. When you modify the top-level items in your application, all other assemblies in the application that reference the top-level assemblies are recompiled as well. In addition, modifying, adding, or deleting certain types of files within the application's known folders will cause the application to restart. The following actions will cause an application restart: Adding, modifying, or deleting assemblies from the application's Bin folder. Adding, modifying, or deleting localization resources from the App_GlobalResources or App_LocalResources folders. Adding, modifying, or deleting the application's Global.asax file. Adding, modifying, or deleting source code files in the App_Code directory. Adding, modifying, or deleting Profile configuration. Adding, modifying, or deleting Web service references in the App_WebReferences directory. Adding, modifying, or deleting the application's Web.config file. When an application restart is required, ASP.NET will serve all pending requests from the existing application domain and the old assemblies before restarting the application domain and loading the new assemblies. HTTP Modules The ASP.NET application life cycle is extensible through IHttpModule classes. ASP.NET includes several classes that implement IHttpModule, such as the SessionStateModule class. You can also create your own classes that implement IHttpModule. If you add modules to your application, the modules themselves can raise events. The application can subscribe to in these events in the Global.asax file by using the convention modulename_eventname. For example, to handle the Authenticate event raised by a FormsAuthenticationModule object, you can create a handler named FormsAuthentication_Authenticate. The SessionStateModule class is enabled by default in ASP.NET. All session events are automatically wired up as Session_event, such as Session_Start. The Start event is raised each time a new session is created. Click here to submit your project requirements to Option Matrix, India. Back to top ASP.NET Application Life Cycle for IIS 7.0 The IIS 7.0 integrated pipeline is a unified request processing pipeline that supports both native-code and managed-code modules. Managed-code modules that implement the IHttpModule interface have access to all events in the request pipeline. For example, a managed-code module can be used for ASP.NET forms authentication for both ASP.NET Web pages (.aspx files) and HTML pages (.htm or .html files). This is true even though HTML pages are treated as static resources by IIS and ASP.NET. For more information about IIS 7.0 Integrated mode,. Architectural Overview A request in IIS 7.0 Integrated mode passes through stages that are like the stages of requests for ASP.NET resources in IIS 6.0. However, in IIS 7.0, these stages include several additional application events, such as the MapRequestHandler, LogRequest, and PostLogRequest events. The main difference in processing stages between IIS 7.0 and IIS 6.0 is in how ASP.NET is integrated with the IIS server. In IIS 6.0, there are two request processing pipelines. One pipeline is for native-code ISAPI filters and extension components. The other pipeline is for managed-code application components such as ASP.NET. In IIS 7.0, the ASP.NET runtime is integrated with the Web server so that there is one unified request processing pipeline for all requests. For ASP.NET developers, the benefits of the integrated pipeline are as follows: The integrated pipeline raises all the events that are exposed by the HttpApplication object, which enables existing ASP.NET HTTP modules to work in IIS 7.0 Integrated mode. Both native-code and managed-code modules can be configured at the Web server, Web site, or Web application level. This includes the built-in ASP.NET managed-code modules for session state, forms authentication, profiles, and role management. Furthermore, managed-code modules can be enabled or disabled for all requests, regardless of whether the request is for an ASP.NET resource like an .aspx file. Managed-code modules can be invoked at any stage in the pipeline. This includes before any server processing occurs for the request, after all server processing has occurred, or anywhere in between. You can register and enable or disable modules through an application's Web.config file. The following illustration shows the configuration of an application's request pipeline. The example includes the following: The Anonymous native-code module and the Forms managed-code module (which corresponds to FormsAuthenticationModule). These modules are configured, and they are invoked during the Authentication stage of the request. The Basic native-code module and the Windows managed-code module (which corresponds to WindowsAuthenticationModule). They are shown, but they are not configured for the application. The Execute handler stage, where the handler (a module scoped to a URL) is invoked to construct the response. For .aspx files, the PageHandlerFactory handler is used to respond to the request. For static files, the native-code StaticFileModule module responds to the request. The Trace native-code module. This is shown, but it is not configured for the application. The Custom module managed-code class. It is invoked during the Log request stage. For information about known compatibility issues with ASP.NET applications that are being migrated from earlier versions of IIS to IIS 7.0. Life Cycle Stages The following table lists the stages of the ASP.NET application life cycle with Integrated mode in IIS 7.0. Stage Description A request is made for an application resource. The life cycle of an ASP.NET application starts with a request sent by a browser to the Web server. In Classic mode in IIS 7.0 and in IIS 6.0, the ASP.NET request pipeline is separate from the Web server pipeline. Modules apply only to requests that are routed to the ASP.NET ISAPI extension. If the file-name extension of the requested resource type is not explicitly mapped to ASP.NET, ASP.NET functionality is not invoked for the request because the request is not processed by the ASP.NET runtime. In integrated mode in IIS 7.0, a unified pipeline handles all requests. When the integrated pipeline receives a request, the request passes through stages that are common to all requests. These stages are represented by the RequestNotification enumeration. All requests can be configured to take advantage of ASP.NET functionality, because that functionality is encapsulated in managed-code modules that have access to the request pipeline. For example, even though the .htm file-name extension is not explicitly mapped to ASP.NET, a request for an HTML page still invokes ASP.NET modules. This enables you to take advantage of ASP.NET authentication and authorization for all resources. App_Code Contains source code for utility classes and business objects (for example, .cs, .vb, and .jsl files) that you want to compile as part of your application. In a dynamically compiled application, ASP.NET compiles the code in the App_Code folder on the initial request to your application. Items in this folder are then recompiled when any changes are detected. Note: Arbitrary file types can be placed in the App_Code folder to create strongly typed objects. For example, placing Web service files (.wsdl and .xsd files) in the App_Code folder creates strongly typed proxies. Code in the App_Code folder is referenced automatically in your application. In addition, the App_Code folder can contain subdirectories of files that need to be compiled at run time. The unified pipeline receives the first request for the application. When the unified pipeline receives the first request for any resource in an application, an instance of the ApplicationManager class is created, which is the application domain that the request is processed in. Application domains provide isolation between applications for global variables and enable each application to be unloaded separately. In the application domain, an instance of the HostingEnvironment class is created, which provides access to information about the application, such as the name of the folder where the application is stored. During the first request, top-level items in the application are compiled if required, which includes application code in the App_Code folder. You can include custom modules and handlers in the App_Code folder as described in Managed-code Modules in IIS 7.0 later in this topic. Response objects are created for each request. After the application domain has been created and the HostingEnvironment object has been instantiated, application objects such as HttpContext, HttpRequest, and HttpResponse are created and initialized. The HttpContext class contains objects that are specific to the current application request, such as the HttpRequest and HttpResponse objects. The HttpRequest object contains information about the current request, which includes cookies and browser information. The HttpResponse object contains the response that is sent to the client, which includes all the rendered output and cookies. The following are some key differences between IIS 6.0 and IIS 7.0 running in Integrated mode and with the .NET Framework 3.0 or later: The SubStatusCode property of the HttpResponse object is available for setting codes that are useful for failed-request tracing. The Headers property of the HttpResponse object provides access to response headers for the response. Two properties of the HttpContext object, IsPostNotification and CurrentNotification, are used when one event handler handles several HttpApplication events. The Headers and ServerVariables property of the HttpRequest object are write-enabled. An HttpApplication object is assigned to the request After all application objects have been initialized, the application is started by creating an instance of the HttpApplication class. If the application has a Global.asax file, ASP.NET instead creates an instance of the Global.asax class that is derived from the HttpApplication class. It then uses the derived class to represent the application. Note: The first time that an ASP.NET page or process is requested in an application, a new instance of the HttpApplication class is created. However, to maximize performance, HttpApplication instances might be reused for multiple requests. Which ASP.NET modules are loaded (such as the SessionStateModule) depends on the managed-code modules that the application inherits from a parent application. It also depends on which modules are configured in the configuration section of the application's Web.config file. Modules are added or removed in the application's Web.config modules element in the system.webServer section. The request is processed by the HttpApplication pipeline. The following tasks are performed by the HttpApplication class while the request is being processed. The events are useful for page developers who want to run code when key request pipeline events are raised. They are also useful if you are developing a custom module and you want the module to be invoked for all requests to the pipeline. Custom modules implement the IHttpModule interface. In Integrated mode in IIS 7.0, you must register event handlers in a module's Init method. 1. Validate the request, which examines the information sent by the browser and determines whether it contains potentially malicious markup. For more information, see ValidateRequest and Script Exploits Overview. 2. Perform URL mapping, if any URLs have been configured in the UrlMappingsSection section of the Web.config file. 3. Raise the BeginRequest event. 4. Raise the AuthenticateRequest event. 5. Raise the PostAuthenticateRequest event. 6. Raise the AuthorizeRequest event. 7. Raise the PostAuthorizeRequest event. 8. Raise the ResolveRequestCache event. 9. Raise the PostResolveRequestCache event. 10. Raise the MapRequestHandler event. An appropriate handler is selected based on the file-name extension of the requested resource. The handler can be a native-code module such as the IIS 7.0 StaticFileModule or a managed-code module such as the PageHandlerFactory class (which handles .aspx files). 11. Raise the PostMapRequestHandler event. 12. Raise the AcquireRequestState event. 13. Raise the PostAcquireRequestState event. 14. Raise the PreRequestHandlerExecute event. 15. Call the ProcessRequest method (or the asynchronous version IHttpAsyncHandler..::.BeginProcessRequest) of the appropriate IHttpHandler class for the request. For example, if the request is for a page, the current page instance handles the request. 16. Raise the PostRequestHandlerExecute event. 17. Raise the ReleaseRequestState event. 18. Raise the PostReleaseRequestState event. 19.Perform response filtering if the Filter property is defined. 20. Raise the UpdateRequestCache event. 21. Raise the PostUpdateRequestCache event. 22. Raise the LogRequest event. 23. Raise the PostLogRequest event. 24. Raise the EndRequest event. 25. Raise the PreSendRequestHeaders event. 26. Raise the PreSendRequestContent event. Note: The MapRequestHandler, LogRequest, and PostLogRequest events are supported only if the application is running in Integrated mode in IIS 7.0 and with the .NET Framework 3.0 or later. Click here to submit your project requirements to Option Matrix, India. Back to top ASP.NET Page Life Cycle When an ASP.NET page runs, the page goes through a life cycle in which it performs a series of processing steps. These include initialization, instantiating controls, restoring and maintaining state, running event handler code, and rendering. It is important for you to understand the page life cycle so that you can write code at the appropriate life-cycle stage for the effect you intend. Additionally, if you develop custom controls, you must be familiar with the page life cycle in order to correctly initialize controls, populate control properties with view-state data, and run any control behavior code. (The life cycle of a control is based on the page life cycle, but the page raises more events for a control than are available for an ASP.NET page alone.) General Page Life-cycle Stages In general terms, the page goes through the stages outlined in the following table. In addition to the page life-cycle stages, there are application stages that occur before and after a request but are not specific to a page. 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 step, 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. Additionally, during the start step, the page's UICulture property is set. Page initialization During page initialization, controls on the page are available and each control's UniqueID property is set. Any themes are also applied to the page. 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. Validation During validation, the Validate method of all validator controls is called, which sets the IsValid property of individual validator controls and of the page. Postback event handling If the request is a postback, any event handlers are called. Rendering Before rendering, view state is saved for the page and all controls. During the rendering phase, the page calls the Render method for each control, providing a text writer that writes its output to the OutputStream of the page's Response property. Unload Unload is called 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 any cleanup is performed. Life-cycle Events 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 (or if it is not defined, since by default it is true), page events are automatically bound to methods that use the naming convention of Page_event, such as Page_Load and Page_Init. For more information on automatic event wire-up. The following table lists the page life-cycle events that you will use most frequently. There are more events than those listed; however, they are not used for most page processing scenarios. Instead, they are primarily used by server controls on the ASP.NET Web page to initialize and render themselves. If you want to write your own ASP.NET server controls, you need to understand more about these stages. Page Event Typical Use PreInit Use this event for the following: Check the IsPostBack property to determine whether this is the first time the page is being processed. Create or re-create dynamic controls. Set a master page dynamically. Set the Theme property dynamically. Read or set profile property values. Note: IDC has been deprecated because it does not provide enough security for data connections. IIS 6.0 will be the last version to include IDC. Init Raised after all controls have been initialized and any skin settings have been applied. Use this event to read or initialize control properties. InitComplete Raised by the Page object. Use this event for processing tasks that require all initialization be complete. PreLoad Use this event if you need to perform processing on your page or control before the Load event. After the Page raises this event, it loads view state for itself and all controls, and then processes any postback data included with the Request instance. Load The Page calls the OnLoad event method on the Page, then recursively does the same for each child control, which does the same for each of its child controls until the page and all controls are loaded. Use the OnLoad event method to set properties in controls and 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 Use this event for tasks that require that all other controls on the page be loaded. PreRender Before this event occurs: The Page object calls EnsureChildControls for each control and for the page. Each data bound control whose DataSourceID property is set calls its DataBind method. For more information   The PreRender event occurs for each control on the page. Use the event to make final changes to the contents of the page or its controls. SaveStateComplete Before this event occurs, ViewState has been saved for the page and for all controls. Any changes to the page or controls at this point will be ignored. Use this event perform tasks that require view state to be saved, but that do not make any changes to controls. 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 that is sent 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. A user control (an .ascx file) automatically incorporates rendering, so you do not need to explicitly render the control in code. Unload This event occurs 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. Additional Page Life Cycle Considerations Individual ASP.NET server controls have their own life cycle that is similar to the page life cycle. For example, a control's Init and Load events occur during the corresponding page events. Although both Init and Load recursively occur on each control, they happen in reverse order. The Init event (and also the Unload event) for each child control occur before the corresponding event is raised for its container (bottom-up). However the Load event for a container occurs before the Load events for its child controls (top-down). You can customize the appearance or content of a control by handling the events for the control, such as the Click event for the Button control and the SelectedIndexChanged event for the ListBox control. Under some circumstances, you might also handle a control's DataBinding or DataBound events. When inheriting a class from the Page class, in addition to handling events raised by the page, you can override methods from the page's base class. For example, you can override the page's InitializeCulture method to dynamically set culture information. Note that when creating an event handler using the Page_event syntax, the base implementation is implicitly called and therefore you do not need to call it in your method. For example, the base page class's OnLoad method is always called, whether you create a Page_Load method or not. However, if you override the page OnLoad method with the override keyword (Overrides in Visual Basic), you must explicitly call the base method. For example, if you override the OnLoad method on the page, you must call base.Load (MyBase.Load in Visual Basic) in order for the base implementation to be run. Catch-up Events for Added Controls If controls are created dynamically at run time or are authored declaratively within templates of data-bound controls, their events are initially not synchronized with those of other controls on the page. For example, for a control that is added at run time, the Init and Load events might occur much later in the page life cycle than the same events for controls created declaratively. Therefore, from the time that they are instantiated, dynamically added controls and controls in templates raise their events one after the other until they have caught up to the event during which it was added to the Controls collection. In general, you do not need to be concerned about this unless you have nested data-bound controls. If a child control has been data bound, but its container control has not yet been data bound, the data in the child control and the data in its container control can be out of sync. This is true particularly if the data in the child control performs processing based on a data-bound value in the container control. For example, suppose you have a GridView that displays a company record in each row along with a list of the company officers in a ListBox control. To fill the list of officers, you would bind the ListBox control to a data source control (such as SqlDataSource) that retrieves the company officer data using the CompanyID in a query. If the ListBox control's data-binding properties, such as DataSourceID and DataMember, are set declaratively, the ListBox control will try to bind to its data source during the containing row's DataBinding event. However, the CompanyID field of the row does not contain a value until the GridView control's RowDataBound event occurs. In this case, the child control (the ListBox control) is bound before the containing control (the GridView control) is bound, so their data-binding stages are out of sync. To avoid this condition, put the data source control for the ListBox control in the same template item as the ListBox control itself, and do not set the data binding properties of the ListBox declaratively. Instead, set them programmatically at run time during the RowDataBound event, so that the ListBox control does not bind to its data until the CompanyID information is available. Data Binding Events for Data-Bound Controls To help you understand the relationship between the page life cycle and data binding events, the following table lists data-related events in data-bound controls such as the GridView, DetailsView, and FormView controls.