Bob Bedell
bobbedell15 at msn.com
Mon Jul 14 22:23:03 CDT 2003
Hi John,
>That would be (in this scenario)
>the preferred action so a "resume next" would be required.
>From Francesco Balena's "Programming Microsoft Visual Basic.NET" (p.
151-152):
"The ability to mix the old-style error trapping and the structured
exception handling mechanism is
important. Whereas the latter approach makes for a more streamilined way of
dealing with errors, you
shouldn't automatically assume that it is also the best way to deal with
errors in all circumstances. As a
matter of fact, you might argue that structured error handling is LESS
flexible than the mechanisms
based on the time honored On Error GoTo statement because there is no simple
way to duplicate the
functionality of Resume and Resume Next statements using the
Try...Catch...Finally block. If you need
to reexecute the statement that caused the problem or just ignore it and
skip to the statement that
follows, using On Error might simplify your coding noticeably. The bottom
line: use the technique that
suits the problem at hand...."
So, there is no simple replacement for "Resume Next" in .Net, you can
mix-and-match On Error Go To
stuff and structured exception handling, but - here's the kicker - a
routine cannot contain BOTH a
'Try' statement and an 'On Error' or 'Resume' statement. So if you have a
situation where you need to
use 'Resume Next' syntax and you also want to use 'Try' syntax, the only
method I've discovered to
date is to call out to another procedure that uses the old-style error
handling from the Try block. The
VB Err object has been extended in VB.NET with a GetException method that is
used to Throw an
exception back to the calling Try block. I modified my console app below and
added Sub LoadCollection
to demonstrate:
Module Module1
' Declare module-level collection.
Private mCollection As New Collection()
Sub Main()
'Uncomment next line to test for NullReferenceException()
'mCollection = Nothing
Try
Call LoadCollection()
'Uncomment next line to test for "Some other exception found: "
'Console.WriteLine(mCollection(4)) ' IndexOutOfRangeException
'Multiple catch clauses ending with "catch all" to ensure all
'exceptions are handled.
Catch exNRF As NullReferenceException
Console.WriteLine("Collection does not exist")
Catch ex As Exception
Console.WriteLine("Some other exception found: " & ex.ToString())
Finally
'...Runs in all of the above cases. Can clean up objects here.
End Try
'Console.Write(mCollection.Count)
Console.WriteLine("")
Console.WriteLine(">>> Press Enter to terminate the program <<<")
Console.ReadLine()
End Sub
Sub LoadCollection()
Dim oObject1 As New Object()
Dim oObject2 As New Object()
Dim oObject3 As New Object()
' OldStyleErrorHandler vs. Structured Exception Handling
On Error GoTo ErrorHandler
'Assign objects to module-level collection.
mCollection.Add(oObject1, "One")
mCollection.Add(oObject2, "Two")
mCollection.Add(oObject3, "Three")
'Uncomment the next line to test Resume Next.
'mCollection.Add(oObject1, "Three") ' duplicate key value
ErrorHandler:
Select Case Err.Number
Case 91 ' Object referenc not set to an instace of an object.
Console.WriteLine(Err.Number & " " & Err.Description)
Throw Err.GetException 'Throws NullReferenceException back to
Try block.
Case 457 ' Add failed. Duplicatekey value supplied.
Console.WriteLine(Err.Number & " " & Err.Description)
Resume Next
End Select
End Sub
End Module
P.S. The Finally statment is a great place for clean up code because the
code between the Finally
keyword and the End Try keywords is guaranteed to run whether or not the
code in the Try block
threw an exception.
HTH
Bob
>From: <jcolby at colbyconsulting.com>
>Reply-To: Access Developers discussion and problem
>solving<accessd at databaseadvisors.com>
>To: "Access Developers discussion and problem
>solving"<accessd at databaseadvisors.com>
>Subject: RE: [AccessD] OT: VB.Net - general questions
>Date: Mon, 14 Jul 2003 17:18:45 -0400
>
>Bob,
>
>But this brings up my question, suppose that on the first attempt to add an
>object to the collection, it is found? The catch runs, but is processing
>returned to the next line below the error? That would be (in this
>scenario)
>the preferred action so a "resume next" would be required.
>
>John W. Colby
>www.colbyconsulting.com
>
>-----Original Message-----
>From: accessd-bounces at databaseadvisors.com
>[mailto:accessd-bounces at databaseadvisors.com]On Behalf Of Bob Bedell
>Sent: Monday, July 14, 2003 4:40 PM
>To: accessd at databaseadvisors.com
>Subject: Re: [AccessD] OT: VB.Net - general questions
>
>
>Hi John,
>
>Put together a little console app below to demonstrate the little I know
>about
>multiple catch clauses. Instead of using an error number, simply use the
>name of
>an Exception class exception type. I use NullReferenceException (collection
>doesn't
>exist scenario) and ArgumentException (duplcate object key value scenario)
>below.
>
>The Finally keyword handles clean up for all three of the catch statements.
>Not sure
>how to implement it in a "Exit_Here:" style label. Unclear about a "Resume
>Next"
>replacement also. Interested to know what you, or others, might uncover.
>
>Bob
>
>Module Module1
>
> ' Declare module-level collection.
> Private mCollection As New Collection()
>
> Sub Main()
>
> Dim oObject1 As New Object()
> Dim oObject2 As New Object()
> Dim oObject3 As New Object()
>
> ' Un-comment next line to test for NullReferenceException()
> 'mCollection = Nothing
>
> Try
>
> ' Assign objects to module-level collection.
> mCollection.Add(oObject1, "One")
> mCollection.Add(oObject2, "Two")
> mCollection.Add(oObject3, "Three")
>
> 'Uncomment next line to test for ArgumentException
> 'mCollection.Add(oObject1, "Three") ' duplicate key value
>
> 'Multiple catch clauses ending with "catch all" to ensure all
> ' exceptions are handled.
> Catch exNRF As NullReferenceException
> Console.WriteLine("Collection does not exist")
> Catch exARG As ArgumentException
> Console.WriteLine("Duplicate key value found")
> Catch ex As Exception
> Console.WriteLine("Some other exception found: " & ex.ToString())
> Finally
> '...Runs in all of the above cases. Can clean up objects here.
> End Try
>
> Console.WriteLine("")
> Console.WriteLine(">>> Press Enter to terminate the program <<<")
> Console.ReadLine()
>
> End Sub
>
>End Module
>
> >From: <jcolby at colbyconsulting.com>
> >Reply-To: Access Developers discussion and problem
> >solving<accessd at databaseadvisors.com>
> >To: "VBA" <dba-vb at databaseadvisors.com>, "AccessD"
> ><AccessD at databaseadvisors.com>
> >Subject: [AccessD] OT: VB.Net - general questions
> >Date: Mon, 14 Jul 2003 14:16:56 -0400
> >
> >I am porting my SysVars class to VB.Net just as an exercise. Of course I
> >use the error handler insertion wizard for Access, but VB.Net has the new
> >Try/catch error handling (which I very much like BTW). But it does mean
> >that no port is trivial since I have to remove all the OnError / resume
> >kind
> >of stuff in every function.
> >
> >In my old code I have a case statement where I accumulated the various
> >errors that could occur, and once handled a resume next would take me
>back
> >into the code. that obviously has to change but I'm a little confused as
> >to
> >what it will change to. For example I have code that attempts to add an
> >object to a collection:
> >
> > mcolObjNames.Add(strObjName, strObjName)
> >
> >There could be two problems here, the first is that no collection has
>been
> >instantiated yet, the second is that the object is already in the
> >collection. Thus a select case would be nice, which was how the old
>error
> >handler in Access worked.
> >
> >Select case err
> > case XXX
> > case YYY
> > Case else
> >end select
> >
> >Now we have:
> >
> >Try
> >
> > mcolObjNames.Add(strObjName, strObjName)
> >
> >Catch e as XXXX (FIRST PROBLEM - WHAT IS xxxx?)
> > Handle error here
> >Finally
> >
> >End Try
> >
> >It certainly looks like e could be used in a select case statement.
> >However
> >to do so I need the equivalent of the "resume next".
> >
> >
> >AND FINALLY...
> >
> >In access I also used a label for the exit such that all exiting code
>could
> >go through the exit point for cleanup of pointers etc. The books I have
> >don't ever show such a construct.
> >
> >
> >John W. Colby
> >www.colbyconsulting.com
>
>
>_______________________________________________
>AccessD mailing list
>AccessD at databaseadvisors.com
>http://databaseadvisors.com/mailman/listinfo/accessd
>Website: http://www.databaseadvisors.com
>From: <jcolby at colbyconsulting.com>
>Reply-To: Access Developers discussion and problem
>solving<accessd at databaseadvisors.com>
>To: "Access Developers discussion and problem
>solving"<accessd at databaseadvisors.com>
>Subject: RE: [AccessD] OT: VB.Net - general questions
>Date: Mon, 14 Jul 2003 17:18:45 -0400
>
>Bob,
>
>But this brings up my question, suppose that on the first attempt to add an
>object to the collection, it is found? The catch runs, but is processing
>returned to the next line below the error? That would be (in this
>scenario)
>the preferred action so a "resume next" would be required.
>
>John W. Colby
>www.colbyconsulting.com
>
>-----Original Message-----
>From: accessd-bounces at databaseadvisors.com
>[mailto:accessd-bounces at databaseadvisors.com]On Behalf Of Bob Bedell
>Sent: Monday, July 14, 2003 4:40 PM
>To: accessd at databaseadvisors.com
>Subject: Re: [AccessD] OT: VB.Net - general questions
>
>
>Hi John,
>
>Put together a little console app below to demonstrate the little I know
>about
>multiple catch clauses. Instead of using an error number, simply use the
>name of
>an Exception class exception type. I use NullReferenceException (collection
>doesn't
>exist scenario) and ArgumentException (duplcate object key value scenario)
>below.
>
>The Finally keyword handles clean up for all three of the catch statements.
>Not sure
>how to implement it in a "Exit_Here:" style label. Unclear about a "Resume
>Next"
>replacement also. Interested to know what you, or others, might uncover.
>
>Bob
>
>Module Module1
>
> ' Declare module-level collection.
> Private mCollection As New Collection()
>
> Sub Main()
>
> Dim oObject1 As New Object()
> Dim oObject2 As New Object()
> Dim oObject3 As New Object()
>
> ' Un-comment next line to test for NullReferenceException()
> 'mCollection = Nothing
>
> Try
>
> ' Assign objects to module-level collection.
> mCollection.Add(oObject1, "One")
> mCollection.Add(oObject2, "Two")
> mCollection.Add(oObject3, "Three")
>
> 'Uncomment next line to test for ArgumentException
> 'mCollection.Add(oObject1, "Three") ' duplicate key value
>
> 'Multiple catch clauses ending with "catch all" to ensure all
> ' exceptions are handled.
> Catch exNRF As NullReferenceException
> Console.WriteLine("Collection does not exist")
> Catch exARG As ArgumentException
> Console.WriteLine("Duplicate key value found")
> Catch ex As Exception
> Console.WriteLine("Some other exception found: " & ex.ToString())
> Finally
> '...Runs in all of the above cases. Can clean up objects here.
> End Try
>
> Console.WriteLine("")
> Console.WriteLine(">>> Press Enter to terminate the program <<<")
> Console.ReadLine()
>
> End Sub
>
>End Module
>
> >From: <jcolby at colbyconsulting.com>
> >Reply-To: Access Developers discussion and problem
> >solving<accessd at databaseadvisors.com>
> >To: "VBA" <dba-vb at databaseadvisors.com>, "AccessD"
> ><AccessD at databaseadvisors.com>
> >Subject: [AccessD] OT: VB.Net - general questions
> >Date: Mon, 14 Jul 2003 14:16:56 -0400
> >
> >I am porting my SysVars class to VB.Net just as an exercise. Of course I
> >use the error handler insertion wizard for Access, but VB.Net has the new
> >Try/catch error handling (which I very much like BTW). But it does mean
> >that no port is trivial since I have to remove all the OnError / resume
> >kind
> >of stuff in every function.
> >
> >In my old code I have a case statement where I accumulated the various
> >errors that could occur, and once handled a resume next would take me
>back
> >into the code. that obviously has to change but I'm a little confused as
> >to
> >what it will change to. For example I have code that attempts to add an
> >object to a collection:
> >
> > mcolObjNames.Add(strObjName, strObjName)
> >
> >There could be two problems here, the first is that no collection has
>been
> >instantiated yet, the second is that the object is already in the
> >collection. Thus a select case would be nice, which was how the old
>error
> >handler in Access worked.
> >
> >Select case err
> > case XXX
> > case YYY
> > Case else
> >end select
> >
> >Now we have:
> >
> >Try
> >
> > mcolObjNames.Add(strObjName, strObjName)
> >
> >Catch e as XXXX (FIRST PROBLEM - WHAT IS xxxx?)
> > Handle error here
> >Finally
> >
> >End Try
> >
> >It certainly looks like e could be used in a select case statement.
> >However
> >to do so I need the equivalent of the "resume next".
> >
> >
> >AND FINALLY...
> >
> >In access I also used a label for the exit such that all exiting code
>could
> >go through the exit point for cleanup of pointers etc. The books I have
> >don't ever show such a construct.
> >
> >
> >John W. Colby
> >www.colbyconsulting.com
>
>
>_______________________________________________
>AccessD mailing list
>AccessD at databaseadvisors.com
>http://databaseadvisors.com/mailman/listinfo/accessd
>Website: http://www.databaseadvisors.com
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