What is the Custom Error in ASP.NET?
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Answer Posted / chauhan rakesh botad
Introduction
Structured exception handling is a fundamental part of the
CLR and provides .NET programmers with a great way of
managing errors. In addition to the 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 webform
contains a label and a button server control. In the
eventhandler for the button click event, the user will be
redirected to another webform Trial.aspx. Since the page
being redirected to is missing ASP.NET it 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:
Now if you click on the button labelled "Generate Error",
you will get the below shown default ASP.NET error page.
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:
1) Off Mode
2) On Mode
3) 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
Customisation 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, customisation 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:
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 = "administrator@domain.com"
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.
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