Error handling in ASP.Net has three aspects:
Tracing - tracing the program execution at page level or application level.
Error handling - handling standard errors or custom errors at page level or application level
Debugging - stepping through the program, setting break points to analyze the code
In this tutorial, we are going to discuss tracing and error handling and we will look into debugging in the next tutorial.
To understand the concepts, create the following sample application. It has a label control, a dropdown list and a link. The dropdown list loads an array list of famous quotes and the selected quote is shown in the label below. It also has a hyperlink which has a nonexistent link.
<%@ Page Language="C#"
AutoEventWireup="true"
CodeBehind="Default.aspx.cs"
Inherits="errorhandling._Default" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
<title>Tracing, debugging and error handling</title>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:Label ID="lblheading" runat="server"
Text="Tracing, Debuggin and Error Handling">
</asp:Label>
<br />
<br />
<asp:DropDownList ID="ddlquotes"
runat="server" AutoPostBack="True"
onselectedindexchanged="ddlquotes_SelectedIndexChanged">
</asp:DropDownList>
<br />
<br />
<asp:Label ID="lblquotes" runat="server">
</asp:Label>
<br />
<br />
<asp:HyperLink ID="HyperLink1" runat="server"
NavigateUrl="mylink.htm">Link to:</asp:HyperLink>
</div>
</form>
</body>
</html>
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The code behind file:
public partial class _Default : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
string[,] quotes =
{
{"Imagination is more important than Knowledge.",
"Albert Einsten"},
{"Assume a virtue, if you have it not",
"Shakespeare"},
{"A man cannot be comfortable without his own
approval", "Mark Twain"},
{"Beware the young doctor and the old barber",
"Benjamin Franklin"},
{"Whatever begun in anger ends in shame",
"Benjamin Franklin"}
};
for (int i=0; i<quotes.GetLength(0); i++)
ddlquotes.Items.Add(new ListItem(quotes[i,0],
quotes[i,1]));
}
}
protected void ddlquotes_SelectedIndexChanged(object sender,
EventArgs e)
{
if (ddlquotes.SelectedIndex != -1)
{
lblquotes.Text = String.Format("{0}, Quote: {1}",
ddlquotes.SelectedItem.Text, ddlquotes.SelectedValue);
}
}
}
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Tracing:
To enable page level tracing, you need to modify the Page directive and add a Trace attribute as:
<%@ Page Language="C#" AutoEventWireup="true"
CodeBehind="Default.aspx.cs"
Inherits="errorhandling._Default"
Trace ="true" %>
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It provides the following information at the top:
The status code sent from the server, each time the page is requested shows the name and time of error if any. The following table shows the common HTTP status codes :
| Number | Description |
|---|
| Informational (100 - 199) |
| 100 | Continue |
| 101 | Switching protocols |
| Successful (200 - 299) |
| 200 | OK |
| 204 | No content |
| Redirection (300 - 399) |
| 301 | Moved permanently |
| 305 | Use proxy |
| 307 | Temporary redirect |
| Client Errors (400 - 499) |
| 400 | Bad request |
| 402 | Payment required |
| 404 | Not found |
| 408 | Request timeout |
| 417 | Expectation failed |
| Server Errors (500 - 599) |
| 500 | Internal server error |
| 503 | Service unavailable |
| 505 | HTTP version not supported |
Error Handling:
Although ASP.Net can detect all runtime errors, still some subtle errors may still be there. Observing the errors by tracing is meant for the developers, not for the users.
Hence, to intercept such occurrence, you can add error handing settings in the web.config file of the application. It is application wide error handling. For example, you can add the following lines in the web.config file:
<configuration>
<system.web>
<customErrors mode="RemoteOnly"
defaultRedirect="GenericErrorPage.htm">
<error statusCode="403" redirect="NoAccess.htm" />
<error statusCode="404" redirect="FileNotFound.htm" />
</customErrors>
</system.web>
<configuration>
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The <customErrors> section has the possible attributes:
To put different custom error pages for different type of errors, the <error> sub tags are used, where different error pages are specified, based of the status code of the errors.
To implement page level error handling, the Page directive could be modified:
<%@ Page Language="C#" AutoEventWireup="true"
CodeBehind="Default.aspx.cs"
Inherits="errorhandling._Default"
Trace ="true"
ErrorPage="PageError.htm" %>
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