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

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