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
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 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>
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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
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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 labeled "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
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>
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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, the
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>
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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>
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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>
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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
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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.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
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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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