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
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 ApplicationManager 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 HostingEnvironment 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.
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 HttpApplicationobject is assigned to the request:
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 this relationship:
The request is processed by theHttpApplicationpipeline:
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.
- Validate the request, which examines the information sent by the browser and determines whether it contains potentially malicious markup. For more information, see ValidateRequest andScript Exploits Overview.
- Perform URL mapping, if any URLs have been configured in the UrlMappingsSection section of the Web.config file.
- Raise the BeginRequest event.
- Raise the AuthenticateRequest event.
- Raise the PostAuthenticateRequest event.
- Raise the AuthorizeRequest event.
- Raise the PostAuthorizeRequest event.
- Raise the ResolveRequestCache event.
- Raise the PostResolveRequestCache event.
- 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.
- Raise the PostMapRequestHandler event.
- Raise the AcquireRequestState event.
- Raise the PostAcquireRequestState event.
- Raise the PreRequestHandlerExecute event.
- 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.
- Raise the PostRequestHandlerExecute event.
- Raise the ReleaseRequestState event.
- Raise the PostReleaseRequestState event.
- Perform response filtering if the Filter property is defined.
- Raise the UpdateRequestCache event.
- Raise the PostUpdateRequestCache event.
- Raise the EndRequest event.
- Raise the PreSendRequestHeaders event.
- Raise the PreSendRequestContent event.
