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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 the very first request for a resource in an ASP.NET application, it instantiates an HttpApplication object. This HttpApplication object then takes over the processing of incoming requests. For the sake of optimization, ASP.NET maintains a pool of HttpApplication objects. When a new HTTP request arrives, ASP.NET uses an object from this pool rather than creating an 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 for application customization through the use of 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 global.asax file. In that case instead of using precompilation the global.asax file will be dynamically compiled at runtime by ASP.NET.

NOTE

The global.asax File Is Protected You use 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 an extension of .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 following predefined event handlers in the global.asax file, as 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 the users of an application.

Application_End()

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

Session_Start()

Called when 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

Table 3.9 shows the event handlers that 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.


You can see from Table 3.9 that 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 page of the application.

STEP BY STEP 3.6 - Handling Global Events Using the global.asax File

  1. Open the global.asax file from the Solution Explorer. Click the Click here to switch to code view hyperlink to switch to the code view.

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

    Sub Application_BeginRequest( _
     ByVal sender As Object, ByVal e As EventArgs)
      ' Store the begin time of the
      ' request in the HttpContext object
      Me.Context.Items.Add("BeginTime", DateTime.Now)

    End Sub

  3. Add the following code to the Application_EndRequest() event handler:

    Sub Application_EndRequest(ByVal sender As Object, _
     ByVal e As System.EventArgs)
      ' Get the begin time from the HttpContext object
      Dim dtBeginTime As DateTime = _
       CType(Me.Context.Items("BeginTime"), DateTime)
    
      ' Calculate the time span between
      ' the start and end of request
      Dim tsProcessingTime As TimeSpan = _
       DateTime.Now.Subtract(dtBeginTime)
    
      ' Display the processing
      ' time taken in the response
      Me.Context.Response.Output.Write("<hr>")
      Me.Context.Response.Output.Write( _
       "{0} took {1} milliseconds to execute.", _
       Me.Request.Url, _
       tsProcessingTime.TotalMilliseconds)

    End Sub

  4. 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.

The modification of the global.asax file in Step by Step 3.6 will affect all other Web Forms in the Web application 305C03. 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 a global.asax file exists 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 the appropriate place to handle global events that are not specific to a Web Form but rather apply to an application as a whole.

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