GetDispParam discriminate b/w VT_EMPTY & zero VT_I4

Misha Koshelev mk144210 at
Tue May 8 00:21:41 CDT 2007

Hi, thank you very much for your help.

I am implementing Installer::RegistryValue for MSI OLE automation and I need a way to tell apart
a VT_EMPTY and a VT_I4 type variant with a value of 0 as these have two different behaviors for this
function under Windows XP. I have not figured out a way to do this with DispGetParam (or VariantChangeType for
that matter; specifically I made a little test to check every single type and find one that could be coerced
to only from a VT_EMPTY but not from a VT_BSTR or a VT_I4 but it did not find any such types; a coercion to a
VT_I4 succeeds and makes a variant of value 0 if coercing either a VT_EMPTY variant or a zero value VT_I4 
variant). I can always just check directly in the DISPPARAMS structure but that seems to be a little dirty if I 
am using DispGetParam for everything else.

Thanks a lot for your advice.


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