Beginning ASP.NET 4.5 in CSharp and VB Opsylum

Exception Handling x 691
‰ On—Every visitor to your site always sees the custom error page when an error occurs.
‰ Off—The custom error page is never shown and full error details are displayed on the page.
‰ RemoteOnly—The full error details are shown to local users (browsing the site from the same
machine as the site runs on), while all other users get to see the custom error page. This setting
enables you to see error messages on your site during development, while your users are
always presented with the custom error page.
Within the opening and closing tags of the customErrors element you define separate
elements, one for each HTTP status code you want to support. The previous configuration defines
two custom pages: one that is shown when the requested page could not be found (a 404 status
code) and one that is shown when a server error occurs (a 500 code).
For all other HTTP status codes you haven’t defined explicitly, the defaultRedirect attribute is
used to determine the custom error page.
The redirectMode attribute determines the way the new page is shown to the user and is discussed
later in this chapter.
Although custom error pages shield your users from the exception details, they don’t help in informing
you that an exception occurred. All these pages do is hide the real error and show a page with a
custom error message instead. To be notified about these exceptions, you could write some code that
looks at the exception and then sends you an e-mail with the details. Alternatively, you could write
code that inserts the error details in a database, or writes them to a text file.
ASP.NET offers you a handy, central location to write code that is triggered when an exception
occurs. You write this code in a special event handler called Application_Error inside the Global.
asax file that you saw first in Chapter 11. Inside this event handler, you can collect relevant data
about the exception, stick it in an e-mail message, and send it to your own Inbox. This gives you
detailed information about exceptions that occur on your site, aiding in fixing the problem as soon
as possible. You see how to write this code in the next exercise.
TRY IT OUT
Handling Exceptions Site Wide
In this Try It Out you learn how to write code in the Global.asax file to send the exception message by
e-mail. Additionally, you see how to create global error pages that are shown to your user in case of an
error.
1. If you didn’t add the Global.asax file to your site in Chapter 11, you need to add it first. To do
so, right-click the website in the Solution Explorer and choose Add Í Add New Item. In the list
with templates, select Global Application Class. Leave its name set to Global.asax and click Add.
2. At the top of the Global.asax file, right after the Application directive, add the following
Import statement. Note that when adding an Import statement in Markup View, both VB.NET
and C# use the keyword Import, rather than Imports and using that you normally use in Code
Behind:
VB.NET