RE: OLE Issue while Porting from Perl 5.6 to Perl 5.8

At 02:10 PM 7/17/2007, Jan Dubois wrote:
>On Tue, 17 Jul 2007, Frank Merrow wrote:> > I suspect this is not so much a Perl 5.8 issue as a "OLE> > Upgrade" issue.> >> > Our Perl 5.6 installation has version 0.1601 of OLE.pm, while the Perl> > 5.8 installation installs 0.1707.> >> > I have an existing COM function that returns a BSTR.> >> > Under Perl 5.6 if the BSTR is "empty" then Perl returned undef.> >> > However, under Perl 5.8 if the BSTR is empty then Perl returns the> > null string ("").>>This is an intentional change in Win32::OLE 0.17 and later. The>vbNullString value from VB is supposed to be treated as an empty string>and not an undefined value. Ideally you should be using VT_NULL and not>VT_BSTR with a NULL pointer to signal an out-of-band value (or VT_ERROR>when you return an error code).

Well I was aware that VB did this, but it was also clear that Perl 
did not . . . so it looks like we just depended on it.

Thanks for the info though, . . .

> > Unfortunately, the software counts on the undef behavior and returned> > empty on error and "" as a valid value (in some cases).> >> > I'm have multiple inherited classes that I am trying to wade my way> > through, but it isn't clear to me at what level the error even occurs.> >> > I was hoping something as simple as Win32::OLE->Options("Variant")> > would fix it, but it doesn't seem to.> >> > If anyone has any insight to how OLE was changed in these versions I> > would appreciate some guidance.>>The only workaround I can think of right now (if you cannot>adjust the COM server)

Well in this case I CAN change the COM back end . . . the problem is 
I want the same behavior under both Perl 5.6 and Perl 5.8.

It doesn't help to fix Perl 5.8 (no customers yet) if I break Perl 
5.6 (several thousand customers).

I wonder if I would get the "current behavior" from BOTH versions of 
Perl if I changed the back end to VT_NULL instead of empty . . . 
something to try.

>is to put the return value into a variant>and then check it:>>     my $retval = $obj->method(@args);>>would become:>>     my $retval = Win32::OLE::Variant->new;>     $obj->Dispatch("method", $retval, @args);>     if ($retval->IsNullString) {>         # V_VT(retval) is VT_BSTR and V_BSTR(retval) is NULL>     }

Okay, this is very help . . . THANK YOU!!!

I figured there had to be a syntax for this, but I just couldn't 
figure it out.  I played with several combinations (none of them 
worked), but obviously not this one.

>This will only work with Win32::OLE 0.17 and later, because the>Win32::OLE::Variant::IsNullString() method doesn't exist in earlier>versions

Yeah, see that is the rub . . . whatever code I eventually move to 
has to work for BOTH versions of Perl.

So far we have done all of our Porting and kept common COM and Perl 
code . . . we would really like to keep that model if at all possible.

Thank you . . . this information has been very helpful.

Frank