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| Visual Basic.NET Error Handling
In VB6, you had the old error handling system of "ON ERROR...". With .NET, you now have the structured error handling system that is similar to that used in C/C++. Below is an example of
how you would implement error handling in VB6.
sub MiscellaneousFunction()
dim objDB as clsDatabase
on error goto errhandler
set objDB = new clsDatabase
' Do some work
exitpoint:
' Clear objects, close any connections.
set objDB = nothing
exit sub
errhandler:
Select case err.number
case 3709
' Display Error Message
case else
' Handle Unknown Errors
End Select
goto exitpoint
end sub
With .NET, you use the following code :-
sub MiscellaneousFunction()
dim objDB as clsDatabase
try
set objDB = new clsDatabase
' Do some work
catch exc as exception
select case exc.number
case 3709
end select
finally
set objDB = nothing
end try
end sub
The golden rule is to replace on error goto errhandler with Try, any tidy up code goes after the finally command. The label that was to be place where the error handling code begins is replaced with
Catch exc as exception. You can different types of exceptions with different Catch statements such as below :
try
catch exc as exception.DivideByZero
' Catch only exceptions cause by dividing by zero.
catch exc as exception
' Catch any exception not already caught.
finally
' Tidy up
end try
All variables declarations must be done before the
try statement other wise you will not be able to do anything with the variable in the tidy up section. The finally clause will be executed whether an error has occurred or not.
To cause an error in VB6, you use the Err.raise followed by an error number, in VB.NET is different, you need to throw an exception. You can either THROW exc as in the
example above or Throw New Exception("") with an error message between the quotes.
VB.NET also enables a user to write an error message to the NT/2000/XP event log, below is example code to write a message to an event log.
dim objLOG as new eventlog
if not eventLog.SourceExists("Source" then
eventlog.createEventSource("Source","Log")
end if
set objLOG = new eventLog()
objLOG.Source="Source"
objLOG.WriteEntry("Msg")
set objLOG = nothing
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