Beginning ASP.NET 4.5 in CSharp and VB Opsylum

684 x CHAPTER 18 EXCEPTION HANDLING, DEBUGGING, AND TRACING
C#
catch (Exception)
{
userMessage = "An unknown error occurred.";
}
The ability to specify an Exception type is useful when you think your code can encounter more
than one exception. In that case, you can have multiple Catch blocks for different Exception types.
The following code is capable of handling a specific SmtpException that may occur during the
mail sending operation, and it’s also capable of catching all other exceptions using its generic Catch
block:
VB.NET
Try
mySmtpClient.Send(myMessage)
Catch smtpException As SmtpException
userMessage = "Sorry, an error occurred while sending your message."
Catch ex As Exception
' Something else went wrong.
End Try
C#
try
{
mySmtpClient.Send(myMessage);
}
catch (SmtpException smtpException)
{
userMessage = "Sorry, an error occurred while sending your message.";
}
catch (Exception ex)
{
// Something else went wrong.
}
The order of the exception-handling blocks is important. .NET scans the list of Catch blocks from
top to bottom and only fires the code in the first block that matches a specific type of exception.
In the preceding example, when an SmtpException occurs (which is a subclass of Exception), it
will be caught by the Catch block that handles exceptions of type SmtpException. Although an
SmtpException is also an Exception, the code in the last Catch block won’t be fired anymore
because only the first matching Catch block is handled. Therefore, if you reverse the order of the
Catch blocks in this example, the more generic Exception block would be executed, and the code in
the SmtpException block would never run.
WARNING The preceding example shows how to catch all exceptions using the
base Exception type in the Catch block. Don’t use this in your own websites.
Instead, handle only those types of exceptions that you know how to deal with,
and let all other, unknown exceptions bubble in the application. Later in this
chapter you see how to centrally handle these unhandled exceptions.