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>

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:

CustErrorHandfig3.gif

Now if you click on the button labeled "Generate Error", you will get the below shown default ASP.Net error page.

CustErrorHandfig5.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:

  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".

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.


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.