Introduction
Structured exception handling is a fundamental part of the CLR and
provides .Net programmers a great way of managing errors. In addition to CLR
exception system, ASP.Net also provides ways of handling errors.
When a runtime or design-time error occurs in an application, ASP.Net shows a
default error page that gives a brief description of the error along with the
line number on which the error occurred. A developer would wish to view this
default error page, during the testing of the application since the description
helps him in rectifying the error. But he would never want a user trying to
access his application, to view this error page. The user would be least
bothered to know about the error. Instead of showing the default error page, it
would be more sensible to show a customized error page that would let the user
send notification of the error to the administrator.
Explanation
Consider an example of an ASP.Net application that generates an error
intentionally to show how ASP.Net detects it and shows the default error page.
The below given web form contains a label and a button server control. In the
event handler for the button click event, the user will be redirected to another
webworm "Trial.aspx". Since the page being redirected to, is missing ASP.Net
will show the default error page indicating it is a runtime error.
Unlike classic ASP, ASP.Net separates the code for the business logic from the
content (i.e. HTML and interface logic). The sample application has two files
named "webform1.aspx" containing the content and "webform1.aspx.vb" containing
the code.
WebForm1.aspx
<%@ Page Language="vb" AutoEventWireup="false" Codebehind="WebForm1.aspx.vb"
Inherits="ErrorSample.WebForm1"%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD
HTML 4.0 Transitional//EN">
<HTML>
<HEAD>
<title></title>
<meta name="GENERATOR" content="Microsoft
Visual Studio.NET 7.0">
<meta name="CODE_LANGUAGE" content="Visual
Basic 7.0">
<meta name="vs_defaultClientScript" content="JavaScript">
<meta name="vs_targetSchema" content="http://schemas.microsoft.com/intellisense/ie5">
</HEAD>
<body MS_POSITIONING="GridLayout">
<form id="Form1" method="post" runat="server">
<asp:Label id="Message" style="Z-INDEX:
101; LEFT: 34px; POSITION: absolute;
TOP:46px"runat="server"></asp:Label>
<asp:Button id="ErrorButton" style="Z-INDEX:
102; LEFT: 268px; POSITION: absolute;
TOP: 41px"runat="server" Text="Generate
Error"></asp:Button>
</form>
</body>
</HTML>
WebForm1.aspx.vb
Public Class WebForm1
Inherits System.Web.UI.Page
Protected WithEvents Message As System.Web.UI.WebControls.Label
Protected WithEvents ErrorButton As System.Web.UI.WebControls.Button
Private Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs)
Handles MyBase.Load
Message.Text = "This sample page generates an Error..."
End Sub
Public Sub ErrorButton_Click(ByVal sender As Object, ByVal e As System.EventArgs)
Handles ErrorButton.Click
Response.Redirect("Trial.aspx")
End Sub
End Class
Now if
you try to run the above web form by viewing it on the browser, you will get the
below shown web page:
![CustErrorHandfig2.gif]()
Now if you click on the button labeled "Generate
Error", you will get the below shown default ASP.Net error page.
![CustErrorHandfig3.gif]()
Customizing Error Page
To customize the default error page, one will have to change the
default configuration settings of the application.
There are three error modes in which an ASP.Net application can work:
-
Off
Mode
-
On
Mode
-
RemoteOnly Mode
The
Error mode attribute determines whether or not an ASP.Net error message is
displayed. By default, the mode value is set to "RemoteOnly".
Off Mode
When the error attribute is set to "Off", ASP.Net uses its default
error page for both local and remote users in case of an error.
On Mode
In case of "On" Mode, ASP.Net uses user-defined custom error page
instead of its default error page for both local and remote users. If a custom
error page is not specified, ASP.Net shows the error page describing how to
enable remote viewing of errors.
RemoteOnly
ASP.Net error page is shown only to local users. Remote requests will
first check the configuration settings for the custom error page or finally show
an IIS error.
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.
![CustErrorHandfig5.gif]()
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.
![CustErrorHandfig6.gif]()
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:
-
Error can be registered as a log entry in the Windows Event Log on the
administrator's machine.
-
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:
![CustErrorHandfig7.gif]()
Description of the error can be viewed by
selecting the appropriate event and double clicking it. Another form pops up as
shown below:
![CustErrorHandfig8.gif]()
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.ToString
mail.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.