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ASP.NET Application

An ASP.NET application is made up of the Web forms, assemblies, and other files stored within a virtual Web directory marked as an IIS application.

When ASP.NET receives a request for a resource in an ASP.NET application, it instantiates an HttpApplication object. The HttpApplication object then takes over the processing of incoming request. An HttpApplication object can only handle one request at a time. To process multiple simultaneous requests, ASP.NET needs to create multiple HttpApplication objects.

For the sake of optimization, instead of destroying the HttpApplication object, ASP.NET maintains a pool of HttpApplication objects. When a new HTTP request arrives, ASP.NET reuses one of the objects from this pool rather than creating a new HttpApplication object from scratch.

The HttpApplication class defines the methods, properties, and events common to all application objects within an ASP.NET application. If you want to customize the behavior of an HttpApplication object, you can derive a class from the HttpApplication class and override the event handlers of the base class for various application level events. An easy way to do this is by using the global.asax file.

The global.asax File

ASP.NET provides an easy way to customize your applications by using the global.asax file. This optional file resides in the root directory of an ASP.NET application. The global.asax file defines a class named Global that derives from the HttpApplication class. When ASP.NET notices that the global.asax file is present for an application, rather than using the implicitly created HttpApplication object, ASP.NET creates instances of the class defined in the global.asax file to handle requests for your application.

Visual Studio .NET automatically creates a global.asax file when you create an ASP.NET Web application project. As with Web forms, Visual Studio .NET creates a code-behind version of the global.asax file. When you make any changes to the code-behind file for global.asax, you must precompile the file before the server can detect those changes. However, it is also possible to create a single file implementation of the global.asax file. In that case, instead of being precompiled, the global.asax file will be dynamically compiled at runtime by ASP.NET.

The global.asax File Is Protected

You use the global.asax file to provide event handlers for various application level events. For security reasons, ASP.NET restricts users of your application from downloading any file with the extension .asax.

Global Event Handlers

The global.asax file is an appropriate place to handle events that are not specific to a Web form, but rather apply to an application as a whole. I'll call these events global events and classify them in two categories—Application and session level events and Per-Request events.

Application and Session Level Events

Application and session level events are fired to signal the start and end of the application or a user session. These events can be handled using the predefined event handlers in the global.asax file shown in Table 3.8.

Table 3.8 Application and Session Level Event Handlers in the global.asax File

Event Handler

Purpose

Application_Start()

Called when an application receives its first request. Generally used to initialize data that is shared among all users of an application.

Application_End()

Called when an application shuts down. Here you can write code to persist the information stored in memory that you want to have reloaded when the application restarts.

Session_Start()

Called when an ASP.NET application creates a new session for a user of the application.

Session_End()

Called when the user's session expires. By default, this happens 20 minutes after the last request of a page from a user.


Per-Request Events

The event handlers shown in Table 3.9 are invoked for each individual page request processed by the HttpApplication object.

Table 3.9 Per-Request Event Handlers

Event Handler

Purpose

Application_BeginRequest()

Called at the beginning of each request.

Application_AuthenticateRequest()

Called when a security module has established the identity of the user.

Application_AuthorizeRequest()

Called when a security module has verified user authorization.

Application_ResolveRequestCache()

Called to resolve the current request by providing content from a cache.

Application_AcquireRequestState()

Called to associate the current request with the session state.

Application_PreRequestHandlerExecute()

Called when ASP.NET begins executing a page.

Application_PostRequestHandlerExecute()

Called when ASP.NET finishes executing a page.

Application_ReleaseRequestState()

Called to save the current state data.

Application_UpdateRequestCache()

Called to update a cache with the responses.

Application_EndRequest()

Called at the end of each request.


As you can see from Table 3.9, you have complete control over how a request is processed. You can write code in any of these event handlers to modify the default behavior of ASP.NET. Step by Step 3.6 uses the Application_BeginRequest() and Application_EndRequest() methods to determine the time it took for each request to process and append this information with every response.

STEP BY STEP

3.6 Handling Global Events Using the global.asax File

  1. Open the global.asax file from the Solution Explorer. Click on the Click Here to Switch to Code View hyperlink to switch to code view.

  2. Add the following code to the Application_BeginRequest() event handler:

  3. protected void Application_BeginRequest(Object sender,
      EventArgs e)
    {
      // Store the begin time of the 
      // request in the HttpContext object
      this.Context.Items.Add("BeginTime", DateTime.Now);
    }
  4. Add the following code to the Application_EndRequest() event handler:

  5. protected void Application_EndRequest(Object sender,
      EventArgs e)
    {
       // Get the begin time from the HttpContext object
       DateTime dtBeginTime = 
       (DateTime) this.Context.Items["BeginTime"];
    
      // Calculate the time span between 
      // the start and end of request
      TimeSpan tsProcessingTime = 
        DateTime.Now-dtBeginTime;
    
      // Display the processing time taken 
      // by the response
      this.Context.Response.Output.Write("<hr>");
      this.Context.Response.Output.Write(
       "{0} took {1} milliseconds to execute.",
       this.Request.Url, 
       tsProcessingTime.TotalMilliseconds);
    }
  6. Run the project. You should see that the page shows a message at the bottom indicating the processing time of the request, as shown in Figure 3.7.

Figure 3.7Figure 3.7 The global.asax file gives you access to application-level events that affect all the pages in an application.

In Step by Step 3.6, the Context object is used to store the begin time. The Context object exposes a key-value collection via the Items property in which you can add values that will be available for the life of the current request. The Context object gives access to the current HTTP request and response.

The modification of the global.asax file in Step by Step 3.6 will affect all other Web forms in the Web application 315C03. If at a later stage, you would like to disable the output generated by the global events, just comment the corresponding lines in the global.asax file.

REVIEW BREAK

  • ASP.NET maintains a pool of HttpApplication objects. When a new HTTP request arrives, ASP.NET uses one of the objects from this pool rather than creating an HttpApplication object from scratch.

  • The global.asax file can be used to define a customized HttpApplication class. When the global.asax file is defined in the Web application's root directory, ASP.NET uses the HttpApplication-derived class defined in the global.asax file to serve the Web application.

  • The global.asax file is an appropriate place for handling global events that are not specific to a Web form, but rather apply to an application as a whole.

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