Custom Error Handling in ASP.NET

Configuration File

Customization of error page can be implemented by adding a value for an attribute "defaultRedirect" in the <customErrors> tag of the configuration file "web.config". This file determines configuration settings for the underlying application.

• Off Mode
  In this scenario, set the mode attribute value to "Off" as shown below:
  
  Web.Config File
  
  <?xml version="1.0" encoding="utf-8" ?>
  <configuration>
  <system.web>
  <customErrors mode="Off" />
  </system.web>
  </configuration>
  
  When the sample ASP.Net web page is viewed in the browser from the remote machine, one gets the below shown default error page.
  
  
  
  The above example thus shows that, whether it is local or remote access, ASP.Net error page is shown.
  
  On Mode
  
  In this scenario, set the mode attribute value to "On" as shown below:
  
  Web.Config File
  <?xml version="1.0" encoding="utf-8" ?>
  <configuration>
  <system.web>
  <customErrors defaultRedirect="error.htm" mode="On" />
  </system.web>
  </configuration>
  
  As shown in the configuration file, the "defaultRedirect" attribute has been set to a user-defined page "error.htm". The user-defined error page can be an ASP.Net web page, classic ASP page or a simple HTML page.
  
  For example, the contents of the user-defined error page "error.htm" can be given as follows:
  
  Error.htm
  
  <HTML>
  <BODY>
  <b>
  We are very sorry for the inconvenience caused to you...<br>
  </b>
  </BODY>
  </HTML>
  
  When the sample ASP.Net web page is viewed in the browser from the remote/local machine, one gets the below shown custom error page.
  
  
  

• RemoteOnly Mode
  
  In this scenario, set the mode attribute value to "RemoteOnly" as shown below:
  
  Web.Config File
  <?xml version="1.0" encoding="utf-8" ?>
  configuration>
  <system.web>
  <customErrors defaultRedirect="error.htm" mode="RemoteOnly" />
  </system.web>
  </configuration>
  
  Since the "defaultRedirect" attribute has been set, if the page is requested from a remote machine page is redirected to "error.htm" and if the page is requested from the local machine the default error page is shown.

Notification of Error to the Administrator

In a practical web application, customization of error pages is not the only requirement. The error, if encountered, should be reported to the administrator so that it can be rectified thus enabling subsequent requests to work properly without any error.

Notification of the error can be sent to the administrator in one of the following two ways:

1. Error can be registered as a log entry in the Windows Event Log on the administrator's machine.
2. An Email can be sent to the administrator with a suitable error message.

• Writing to the Event Log
  
  In ASP.Net, error can be handled programmatically by writing appropriate code in the page-level error event, for errors on an individual page or in the application-level error event for handling errors that may occur in any page of the application.
  
  Therefore, code for writing in the Event Log should be written in either of the events, depending on the requirement of the application. To illustrate this example, I have written the code in the application-level event with the error mode set to "RemoteOnly" and the "defaultRedirect" attribute to "error.htm". The application-level error event should be included in the global file "global.asax" within the same application folder.
  
  The contents of the global file can be given as follows:
  
  Writing Log Entry in the Event Log
  
  Imports System.Web
  Imports System.Web.SessionState
  Imports System.Diagnostics
  Public Class Global
  Inherits System.Web.HttpApplication
  Sub Application_Error(ByVal sender As Object, ByVal e As EventArgs)
  Dim ErrorDescription As String = Server.GetLastError.ToString
  'Creation of event log if it does not exist
  Dim EventLogName As String = "ErrorSample"
  If (Not EventLog.SourceExists(EventLogName)) Then
  EventLog.CreateEventSource(EventLogName, EventLogName)
  End If
  ' Inserting into event log
  Dim Log As New EventLog
  Log.Source = EventLogName
  Log.WriteEntry(ErrorDescription, EventLogEntryType.Error)
  End Sub
  End Class
  
  Event Log support is provided in .Net through the namespace "System.Diagnostics". So, for the above code to work, it is very essential to add a reference to the above-mentioned namespace in the project. In the event handler for application-level error, a log named "ErrorSample" is created if it does not exist in the Event Log. If it already exists, the error entry is added to the existing list of events. After viewing the page on the browser from a remote machine, the event will get listed in the Event Log on the administrator's machine as shown below:
  
  
  
  Description of the error can be viewed by selecting the appropriate event and double clicking it. Another form pops up as shown below:
  
  
  
  
• Sending an Email to the Administrator
  
  To illustrate this example, I have written the code for sending an Email to the administrator in the application-level error event. The contents of the global file can be given as follows:
  
  Sending Email To the Administrator
  
  Imports System.Web
  Imports System.Web.SessionState
  Imports System.Web.Mail
  Public Class Global
  Inherits System.Web.HttpApplication
  Sub Application_Error(ByVal sender As Object, ByVal e As EventArgs)
  Dim mail As New MailMessage
  Dim ErrorMessage = "The error description is as follows : "
  & Server.GetLastError.ToStringmail.To = [email protected]
  mail.Subject = "Error in the Site"
  mail.Priority = MailPriority.High
  mail.BodyFormat = MailFormat.Text
  mail.Body = ErrorMessage
  SmtpMail.Send(mail)
  End Sub
  End Class
  
  In the above code, SMTP service is being used to send the mail across. SMTP mail service support is provided in .Net through the namespace "System.Web.Mail". So, for the above code to work, it is very essential to add a reference to the above-mentioned namespace in the project.