Complete Example for Error Handlers

The following example shows the Web.config file. The customErrors section specifies how to handle errors that occur with file types that are mapped to ASP.NET, such as .aspx, .asmx, and .ashx files. (In IIS 6.0 and in IIS 7.0 in classic mode, static content files such as .html and .jpg files are not mapped to ASP.NET.)

The settings in the example customErrors section cause any unhandled HTTP 404 (file not found) errors to be directed to the Http404ErrorPage.aspx file. These HTTP 404 errors would occur if a request were made for an .aspx file, .asmx file, and so on and if the requested file did not exist. All other unhandled errors in ASP.NET files are directed to the DefaultRedirectErrorPage.aspx file.

If static content files are not handled by ASP.NET, a request for a nonexistent .html or .jpg file does not cause a redirect to the Http404ErrorPage.aspx file. If you want ASP.NET to handle requests for all file types, you can configure IIS to map file-name extensions to ASP.NET.

Note
In the example, the mode attribute is set to "On" so that you can error messages when you run the example in Visual Studio. In a production environment, this setting would normally be "RemoteOnly". ASP.NET then renders error pages to external users. If a request is made on the server computer (localhost), ASP.NET renders a page with detailed error information.

<configuration>
  <appSettings/>
  <connectionStrings/>
  <system.web>
    <compilation debug="true" />

    <!-- Turn on Custom Errors -->
    <customErrors mode="On" 
      defaultRedirect="DefaultRedirectErrorPage.aspx">
      <error statusCode="404" redirect="Http404ErrorPage.aspx"/>
    </customErrors>

  </system.web>
</configuration>

The following example shows the Global.asax file

Security Note
Never set the mode attribute in the customErrors element to "Off" in the Web.config file if you have not created an Application_Error handler in the Global.asax file. If the mode is set to "Off," potentially compromising information about your Web site can be exposed to anyone who can cause an error to occur on your site.

The following example shows the ExceptionUtility file. Error logs might be directed to the computer's ErrorLog file, or, if the computer is part of a Web farm, the error log might be recorded in a globally available text file, or even a database. You might also need to immediately notify system administrators of a problem.

The ExceptionUtility class in the example has two static methods: one to log the exception, and one to notify system administrators. How those methods are implemented in your code depends on the needs of your organization. For this example, you must grant write permissions to the ASP.NET worker process account (by default, this is NETWORK SERVICE) for the App_Data folder to enable the application to write to the error log.

The following example shows the Default.aspx page. This file provides several buttons, each of which raises a different exception. The Page_Error handler on the page displays an error page and logs some of these errors. Unhandled errors are passed to the Application_Error handler in the Global.asax file. The Application_Error handler displays an error page and logs some of the remaining errors. Any errors that are still not handled are directed to the page indicated by the customErrors section of Web.config file.

The following example shows the GenericErrorPage.aspx page. This page creates a safe message that it displays to remote users. For local users (typically developers and testers of the application), the page displays a complete exception report. The Page_Error handler redirects InvalidOperationException errors to this page.

The following example shows the HttpErrorPage.aspx page. This page also creates a safe message that depends on the value of the error code, which it displays to remote users. For local users, the page displays a complete exception report. The Application_Error handler redirects HttpException errors to this page.

The following example shows the Http404ErrorPage.aspx page. ASP.NET redirects unhandled HTTP 404 (file not found) errors to this page. The page displays the same message to remote and local users.

The following example shows the DefaultRedirectErrorPage.aspx page. ASP.NET redirects any unhandled errors except HTTP 404 errors to this page. The page displays the same message to remote and local users.