| Error handling in Visual Basic 6.0 largely consists of On Error
functionality such as On Error Resume Next and On Error Goto. While
sufficient to create basic error handling, they can be fiddly to use, especially on blocks
of code that the developer knows is quite likely to raise errors on occasions (such as a
disk resource or a database connection being unavailable). Even more basic is the error
handling in classic ASP's VBScript, aside from an On Error Resume Next facility
there was not much of any use in error handling, and even then the On Error Resume Next
functionality did not always work reliably, especially when used within Functions.
Happily, the error handling capabilities of the .NET Framework's VB.NET programming
language are greatly enhanced compared to Visual Basic 6.0 and VBScript.
Handling faulty VB.NET code
The following ASP.NET sample code loads a user control from a file and adds it to a
PlaceHolder control:
Dim UserControlWeeklyOffers As Control = LoadControl("WeeklyOffers.ascx")
PlaceHolder1.Controls.Add(UserControlWeeklyOffers)
If the user control file WeeklyOffers.ascx does not exist, then the following
ASP.NET error will be displayed in the web browser:
The file '/WeeklyOffers.ascx' does not exist.
However, if the code is enclosed within a Try ... Catch statement, then the page will
be displayed in the web browser, but the user control will not be loaded into the
PlaceHolder control.
Try
Dim UserControlWeeklyOffers As Control = LoadControl("WeeklyOffers.ascx")
PlaceHolder1.Controls.Add(UserControlWeeklyOffers)
Catch
Response.Write("Could not load weekly
offers.")
End Try
In this example, if the WeeklyOffers.ascx file does not exist then the Could
not load weekly offers message will be displayed instead.
Note that the Catch statement is not required, so the following code may also be used:
Getting full details of an Exception
The Try ... Catch block can be extended to give further details of the exception
generated by the failing code. In the sample below an Exception object, Ex is
instantiated if the code between the Try and Catch statements encounters an error:
Try
Dim UserControlWeeklyOffers As Control = LoadControl("WeeklyOffers.ascx")
PlaceHolder1.Controls.Add(UserControlWeeklyOffers)
Catch Ex As
Exception
Response.Write("Could not load weekly offers.
")
Response.Write("Due to: " &
Ex.Message)
End Try
Using the Finally statement
The Finally statement can be used denote code that needs to run regardless of
whether the code in the Try code block was successfully executed. In the example below, a Wednesday
Offers control is loaded regardless of whether or not the Weekly Offers
control was loaded:
Try
Dim UserControlWeeklyOffers As Control = LoadControl("WeeklyOffers.ascx")
PlaceHolder1.Controls.Add(UserControlWeeklyOffers)
Catch Ex As
Exception
Response.Write("Could not load weekly offers.
")
Response.Write("Due to: " &
Ex.Message)
Finally
If
DateTime.Now.DayOfWeek = DayOfWeek.Wednesday Then
Dim
UserControlWednesdayOffers As Control = LoadControl("WednesdayOffers.ascx")
PlaceHolder1.Controls.Add(UserControlWednesdayOffers)
End If
End Try
Logging/emailing errors
Once the VB.NET code has been modified to trap exceptions, it is feasible to write a
.NET routine that will log errors or email them to a system administrator. This allows
application developers to monitor application performance, detect possible security issues
and allow them to respond quickly to sudden problems such as the unavailability of a
database server for example.
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