This article describes how to use Visual C# .NET code to trap and respond to errors when they occur in ASP.NET. ASP.NET has improved the error handling options from traditional Microsoft Active Server Pages (ASP). In ASP.NET, you can handle errors at several different levels in your applications.

Back to the top

New features in ASP.NET

ASP.NET offers several advances in how you can handle and respond to errors. In traditional ASP, you handle errors with "On Error Resume Next" (or try-catch blocks in JScript). Alternately, if you are running Microsoft Internet Information Services (IIS) 5.0, you use the ASPError object to create a custom error reporting page. However, these approaches have their limitations.

ASP.NET provides several levels at which you can handle and respond to errors that may occur when you run an ASP.NET application. ASP.NET provides three main methods that allow you to trap and respond to errors when they occur: Page_Error, Application_Error, and the application configuration file (Web.config).

This article demonstrates how to use these new features in your ASP.NET application. Although this article describes how to provide custom error pages and general error reporting as it relates directly to ASP.NET, this article does not describe other error handling approaches such as the try-catch-finally block and the Common Language Runtime (CLR) exception system.

Back to the top

How to use the Page_Error method

The Page_Error event handler provides a way to trap errors that occur at the page level. You can simply display error information (as the sample code to follow does), or you can log the event or perform some other action.

Note This example displays detailed error information in the browser only for demonstration purposes. You will want to be cautious when displaying detailed information to the end user of the application, especially when the application is running on the Internet. A more appropriate action would be to display a message to the user notifying them that an error has occurred, and then actually logging the specific error details in the event log.

This example throws a null exception, which forces an error to occur in the Page_Load event handler. Follow these steps to create the initial page that will demonstrate using the Page_Error event handler.

1.	Follow these steps to add a new file named PageEvent.aspx to your project: a. Open Microsoft Visual Studio .NET. b. In Solution Explorer, right-click the project node, point to Add, and then click Add Web Form. c. In the Name text box, type PageEvent.aspx, and then click Open.
2.	Add the following code to PageEvent.aspx: <script language=C# runat="server"> void Page_Load(object sender, System.EventArgs e) { throw(new ArgumentNullException()); } public void Page_Error(object sender,EventArgs e) { Exception objErr = Server.GetLastError().GetBaseException(); string err = "<b>Error Caught in Page_Error event</b><hr><br>" + "<br><b>Error in: </b>" + Request.Url.ToString() + "<br><b>Error Message: </b>" + objErr.Message.ToString()+ "<br><b>Stack Trace:</b><br>" + objErr.StackTrace.ToString(); Response.Write(err.ToString()); Server.ClearError(); } </script> Note In this code sample, the AutoEventWireup attribute is not explicitly set. If you do not explicitly assign a value to the AutoEventWireup attribute, the default value true is used. If you are using Visual Studio .NET to develop your applications, the Web Form template code explicitly sets the AutoEventWireup attribute value to false. There is an important difference between the default value that ASP.NET uses, and the default value that the Visual Studio .NET template code assigns to this attribute. If the AutoEventWireup attribute value is set to false, the event handlers that are declared in the .ASPX page do not fire. This may be confusing if you do not know about this functionality.
3.	From the File menu, click Save PageEvent.aspx.
4.	Right-click the page, and then click View in Browser to run the page. Notice that the error is thrown and reported according to the code specifications.

Note You may notice that the code issues a call to Server.ClearError. This prevents the error from continuing to the Application_Error event handler.

In addition, you should also take note of the Inherits attribute in the @ Page directive. If Inherits is set, you must build the project before you browse to the page. If you do not build the project first, you receive the following error message:

Back to the top

How to use the Application_Error method

Similar to the Page_Error event handler, you can use the Application_Error event handler to trap errors that occur in your application. Due to the event's application-wide scope, you can log of application error information or handle other application-level errors that may occur.

The sample to follow is based on the preceding Page_Error code sample and would be fired if the error in Page_Load was not trapped in the Page_Error event handler. The Application_Error event handler is specified in the Global.asax file of your application. For simplicity, the steps in this section create a new page in which to throw the exception, trap the error in the Application_Error event handler of the Global.asax file, and write the error to the event log. The following steps demonstrate how to use the Application_Error method:

1.	Add a new file named AppEvent.aspx to your project.
2.	Add the following code to AppEvent.aspx: <script language=C# runat="server"> void Page_Load(object sender, System.EventArgs e) { throw(new ArgumentNullException()); } </script> Note The information discussed in the "Page_Error" section about the AutoEventWireup attribute also applies to the code sample in this step. See the information in the "Page_Error" section for more details.
3.	From the File menu, click Save AppEvent.aspx.
4.	Add the Application_Error event handler to the Global.asax file to trap the error that you throw in the Page_Load event handler of the AppEvent.aspx page. Notice that you must add another using statement for the System.Diagnostics namespace to Global.asax to use the event log. Add the following code to the Global.asax file: using System.Diagnostics; protected void Application_Error(object sender, EventArgs e) { Exception objErr = Server.GetLastError().GetBaseException(); string err = "Error Caught in Application_Error event\n" + "Error in: " + Request.Url.ToString() + "\nError Message:" + objErr.Message.ToString()+ "\nStack Trace:" + objErr.StackTrace.ToString(); EventLog.WriteEntry("Sample_WebApp",err,EventLogEntryType.Error); Server.ClearError(); //additional actions... }
5.	Save the Global.asax file.
6.	In Visual Studio .NET, on the Build menu, click Build.
7.	Right-click the page, and then click View in Browser. In this case the page will be blank, however, you should notice that a new entry has been added in the event log. This sample makes an entry in the Application log, which is accessible from the Event Viewer. After logging the error you might want to redirect the user to another more user-friendly error page, or perform some additional actions if needed.

Back to the top

How to use the Web.config file

If you do not call Server.ClearError or trap the error in the Page_Error or Application_Error event handler, the error is handled based on the settings in the <customErrors> section of the Web.config file. In the <customErrors> section, you can specify a redirect page as a default error page (defaultRedirect) or specify to a particular page based on the HTTP error code that is raised. You can use this method to customize the error message that the user receives.

If an error occurs that is not trapped at any of the previous levels in your application, this custom page is displayed. This section demonstrates how to modify the Global.asax file so that Server.ClearError is never called. As a result, the error is handled in the Web.config file as the last point to trap the error.

1.	Open the Global.asax file from the previous example.
2.	Comment out the Server.ClearError line to ensure that the error surfaces in the Web.config file.
3.	Save your changes to Global.asax. Your code should now appear similar to the following: using System.Diagnostics; protected void Application_Error(object sender, EventArgs e) { Exception objErr = Server.GetLastError().GetBaseException(); string err = "Error Caught in Application_Error event\n" + "Error in: " + Request.Url.ToString() + "\nError Message:" + objErr.Message.ToString() + "\nStack Trace:" + objErr.StackTrace.ToString(); EventLog.WriteEntry("Sample_WebApp",err,EventLogEntryType.Error); //Server.ClearError(); //additional actions... }
4.	Add the following code to the <customErrors> section to redirect the user to a custom page: <customErrors defaultRedirect="http://hostName/applicationName/errorStatus.htm" mode="On"> </customErrors> Note You must modify the file path in defaultRedirect attribute so that it references the relevant Web server and application names.
5.	Because the errors that are trapped at this level are sent to a default error page, you must create an error page named ErrorStatus.htm. Keep in mind that you are using this method to control what is presented to the user, so this example uses an .htm page for the error page. Add the following code to ErrorStatus.htm: <HTML> <HEAD> <TITLE></TITLE> <META NAME="GENERATOR" Content="Microsoft Visual Studio 7.0"> </HEAD> <BODY> <b>Custom Error page!</b> <br> You have been redirected here from the <customErrors> section of the Web.config file. </BODY> </HTML>
6.	To test the code, save the files, build the project, and then view AppEvent.aspx in the browser. Notice that when the error is thrown, you are redirected to the ErrorStatus.htm page.

Although you can reference a default error page in the value of the defaultRedirect attribute in the <customErrors> section, you can also specify a particular page to redirect to based on the HTTP error code that is raised. The <error> child element allows for this option. For example:

<customErrors defaultRedirect="http://hostName/applicationName/errorStatus.htm" mode="On">
	<error statusCode="404" redirect="filenotfound.htm" />
</customErrors>
				

Note The page that is specified in defaultRedirect of the <customErrors> section is an .htm file. I

Notice that the <customErrors> section includes a mode attribute that is set to On. The mode attribute is used to control how the error redirection occurs. For example, if you are developing the application, you most likely want to see the actual ASP.NET error messages and do not want to be redirected to the more user-friendly error page. The mode attribute includes the following settings:

•	On: Unhandled exceptions redirect the user to the specified defaultRedirect page. This mode is used mainly in production.
•	Off: Users receive the exception information and are not redirected to the defaultRedirect page. This mode is used mainly in development.
•	RemoteOnly: Only users who access the site on the local computer (by using localhost) receive the exception information. All other users are redirected to the defaultRedirect page. This mode is used mainly for debugging.

Back to the top

Troubleshooting

In its default installation on Microsoft Windows 2000 and on Microsoft Windows XP, ASP.NET runs Web application code in a worker process. The identity of this process defaults to an unprivileged local account called the ASPNET account. In beta releases of ASP.NET, the process identity was System, a powerful administrative account with many privileges on the machine.

In its default installation on Windows Server 2003 (IIS 6), ASP.NET runs Web application code in a worker process. The identity of this process defaults to a limited account called NetworkService.

For more information about this change and how it can effect running the code in this article, and other other code that might need additional access rights, visit the following Web sites:

Back to the top

For more information, visit the following Microsoft Web sites:

Back to the top