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CHAPTER 11 DEBUGGING AND EXCEPTION HANDLING
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3. Drag-and-drop the stylesheet used in your application onto the web form designer surface to link to it. 4. Switch to the HTML view of CustomError.aspx and add some friendly error message within the <form> element: <form id="CustomError" method="post" runat="server"> <p> <font size="5" color="red"> <img src="~/Images/sad.gif">  <b>An error has been found...</b> </font> </p> <p> We have detected an error in the Friends Reunion website. <br/> If this error persists, please contact our support team... </p> </form> 5. After creating the custom error page, you need to tell ASP .NET to show that error page instead of the default error page. To do this, edit the <customErrors> element of the Web.config file for the application like this: <configuration> <system.web> ... <customErrors defaultRedirect="CustomError.aspx" mode="On" /> ... </system.web> </configuration> 6. Now you need some code that explicitly throws an exception, so you can test that the custom error page works properly. To do this, edit LegalStuff.aspx.vb by adding this code to the Page_Load() method: Private Sub Page_Load(ByVal sender As System.Object, _ ByVal e As System.EventArgs) Handles MyBase.Load Throw New NullReferenceException End Sub 7. Set LegalStuff.aspx as the start page and press Ctrl+F5. A new browser instance will open with the Login page for Friends Reunion. Log in, and you will be automatically redirected to LegalStuff.aspx. You should see the page shown in Figure 11-28.
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CHAPTER 11 DEBUGGING AND EXCEPTION HANDLING
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Figure 11-28. The custom error page
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How It Works When an exception is neither handled by the exception-handling code in the application nor cleared in the Page_Error() or Application_Error() event handlers, ASP.NET will check the <customErrors> element of the Web.config file to see whether a default error page has been specified for your application, through the defaultRedirect attribute. If it finds one, it will transfer execution to that page; if not, it will transfer execution to the default error page, which displays all the information about the unhandled exception. Note that you also changed the mode attribute from RemoteOnly to On, so that you can see the custom error page, even when executing on the local machine. With that new value, the custom page is always shown. If you wanted to still get the detailed, default error page showing the complete exception when running the application locally, you would leave the previous RemoteOnly value.
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Logging Exceptions to the System Event Log
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You have successfully shown the user a customized message. However, in order for your support team (that may be you!) to diagnose the error and determine its cause, you need a way to permanently store the error information for later analysis. Windows provides the event log for that purpose. The event log is a central location, which can also be accessed remotely, that stores logs for all applications that wish to write to it. ASP.NET applications can also do so, by using the System.Diagnostics.EventLog class.
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CHAPTER 11 DEBUGGING AND EXCEPTION HANDLING
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There is one caveat, though: the first time an application logs an event, an event source must be created for it. This task requires administrative privileges, unlike subsequent log actions. For security reasons, the ASP .NET worker process account doesn t have these permissions. Therefore, the first call to write an entry in the event log (and all subsequent ones) will fail because the event source for your application cannot be created. In 13, we will discuss deployment options in .NET and how you can create this event source at installation time. For now, we can use the following workaround: 1. Open the Administrative Tools Computer Management tool. 2. Select the Users node inside Local Users and Groups, from the System Tools node. 3. Double-click the ASPNET account. 4. On the Member Of tab, add the Administrators group. Then click OK in the dialog box. What you have done is increase the account permissions to allow for the automatic creation of the event source the first time you write an entry to the event log. You must restart IIS in order for this change to have effect. You can do so by running the iisreset command (either by using Start Run or the command line). Needless to say, you must remove this permission after you ve completed the next Try It Out ! Now let s see the code to actually write to the event log. Try It Out: Log Exceptions to the System Event Log We mentioned earlier that the Application_Error() event handler is an ideal place for adding application-wide logging of errors. It s ideal because it receives unhandled exceptions raised anywhere in your code, not just on a specific page. Let s see how to code this. 1. Open the Global.asax.vb file, and edit the Application_Error() method as follows: Public Class Global Inherits System.Web.HttpApplication ... Sub Application_Error(ByVal sender As Object, ByVal e As EventArgs) System.Diagnostics.EventLog.WriteEntry("FriendsReunion", _ Server.GetLastError().InnerException.ToString(), _ System.Diagnostics.EventLogEntryType.Error) End Sub ... End Class 2. That s all the code required! Press F5 to run Friends Reunion. Log in to the application, and then navigate to LegalStuff.aspx (which still has the code to throw a NullReferenceException in its Page_Load() method). 3. Open the Event Viewer (Control Panel Administrative Tools Event Viewer) and click the Application log. A list of the last application events recorded should be displayed. Locate the one whose Source column shows FriendsReunion and double-click it. You ll see the log shown in Figure 11-29.
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