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Earlier in this chapter we learned that we could instrument our code to help detect the root cause of problems, and saw how to use Health Monitoring as an early warning system to alert us when our application s health is waning. With all of these safety measures in place, we can now deploy our application, comfortable in the knowledge that we can detect errors before our users do, right Well, unfortunately this is not the case, and in reality even the best applications succumb to errors at times. What we need is a way to handle errors when they occur at runtime and provide a way for our application to handle these errors gracefully, so that users are not left looking at an ASP.NET Error page such as the one shown in figure 9.4. This is typical of what users see when something goes wrong with an ASP.NET page and an unhandled exception occurs. Displaying such a page to users is disconcerting
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Figure 9.4 The dreaded ASP.NET error page will be displayed by default whenever an unhandled exception gets thrown.
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because the page shows code, detailed error messages, and other technical information that users would simply not understand. Showing this level of technical information could be even worse if it were viewed by a visitor with mischievous intentions such as a hacker. A hacker viewing technical details shown in figure 9.4 might gain valuable insight into database connection strings and other information that could then be used to hack the website. Having said all that, the ability to view such detailed information about errors is very useful when we are developing the application, as it helps us find errors and apply fixes faster. So we need to have the ability to flip a switch and have this page displayed in the development environment, but to display a custom error page when the application is deployed into other environments. Thankfully, ASP.NET provides us with this ability. 9.3.1 Providing a custom error page To display a custom error page whenever unhandled errors occur, we can simply use the configuration settings of our application to tell ASP.NET which page to redirect to when errors occur. We do this by using the customErrors configuration element in the web configuration file. The following snippet shows an example of the customErrors element being set so that ErrorPage.aspx will be displayed whenever an unhandled error occurs within the application.
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< xml version="1.0" encoding="utf-8" > <configuration> <system.web> <customErrors mode="On" defaultRedirect="ErrorPage.aspx" /> </system.web> </configuration>
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In this example, the mode attribute is set to On while the defaultRedirect attribute contains the URL of the page where we want to display custom error information. Typically, such a page would inform the users that an error has occurred and that the system administrator has already been notified. Finally, it is useful to provide users with a link from this page that allows them to navigate back into the site such as a link to the home page or a link back to the page that the users have just come from. Listing 9.4 shows a sample custom error page that tells users what happened.
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Listing 9.4 A standard page can be used to display to users in place of the dreaded error page. This page can contain useful information and provide users with a way to continue their browsing experience.
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<html> <body> <p> We are sorry for the inconvenience but an error has occurred. detailed error message has been sent to the system administrator. Please A
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click <a href="javascript: window.history.go(-1);">here</a> to return to the page that you were just at, or click <a href="Default.aspx">here</a> to go our Home Page. </p> </body> </html>
When critical errors occur on our website, users will no longer be subjected to the confusing and highly technical standard ASP.NET error page. Instead, users are shown a page with which they are more familiar, and which helps them to recover and continue using our application. In the case of the page shown in listing 9.4, we have explained that an error has occurred and we are providing users with links which will allow continued use of the site. However, even though we ve provided users with a more friendly error page, we still need a way to log the failure and notify an administrator that it has occurred so that we take steps to fix the problem behind the failure. 9.3.2 Logging the failure The simplest place to put notification code is in the Global Application class. To add a Global Application class to our application, we simply right-click on the Visual Studio solution tree from within our web application and use the Add New Item menu option to add a new Global Application file to our application. This adds a file named Global.asax to our project. In the Global Application class, we can write code that will run when certain application level events occur within the application. For example, whenever a new page is requested within the application, the BeginRequest event is fired and we can write code in the Global Application class to handle that event. Likewise, the Global Application class allows us to handle the Application s Error event, so we can handle this event and write code that runs when an unhandled error occurs. Listing 9.5 is an example of how to handle the Application s Error event from within the Global.asax file and use it to send an email to an administrator notifying them of the error.
Listing 9.5 By logging unhandled errors from within the Applicaion_Error event handler we can get notifications when unexpected failures occur.
void Application_Error(object sender, EventArgs e) { Exception ex = Server.GetLastError(); if (ex != null) { string body = "The following error has occurred: " + ex.ToString() ; string subject = "Application Error."; string from = "ErrorHandler@AdventureWorks.com"; string to = "Adminstrator@AdventureWorks.com";
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