How do I extract data from an OLE variant?

Question

I am trying to retrieve a string from a Dot Net function.  What I get out is a Labview variant.
From my research, I suspect that what Labview is giving me in this variant is a pointer to the string that I need to dereference.  Problem is that I can't even get the pointer value out of the variant.
If I view the variant as an indicator, I see that it has a value but no matter what type I wire to the Variant to Data function, I get a type mismatch error.
The value is there, why can't Labview give it to me?

Answer 1

MoveBlock() won't help you here much. VT_DISPATCH indeed contains a pointer, but not a pointer to data but instead a pointer to an IDispatch COM object. This is basically a pointer to an object containing internally one (and typically more) so called COM virtual method dispatch tables. The only safe way to get at the information is by using the COM QueryInterface() method in that object to retrieve the IDispatch interface, then calling the IDispatch methods to inspect the data contents the object contains and then retrieving the actual data.
This is impossible to do on your own in a LabVIEW diagram but requires quite a bit of C(++) code. However if you know what the IDispatch is really holding internally (and your VBA.ICollection sounds like a plausible idea) then you can create a compatible Automation refnum by dropping one on your front panel and then browsing to the according ActiveX object type (here most like in the Visual Basic typelibrary) and then using Variant to Data, let LabVIEW comvert the contained IDispatch to the actual Automation refnum to the data object. Variant to Data will return an error if you selected the wrong ActiveX refnum type.
An VBA.ICollection is however not a string or numeric data already but more like an Array like container, so you would then have to use the VBA.ICollection methods to enumerate the elements in it as variants and convert them to the actual datatype using Variant to Data again.
Also note that pointers are 32 bit in LabVIEW 32 Bit and 64 bit in LabVIEW for 64 bit. This is independent if you run on 32 bit or 64 bit Windows, since a 32 Bit version of LabVIEW runs in a 32 bit environment even on 64 bit Windows.
Rolf Kalbermatter
CIT Engineering Netherlands
a division of Test & Measurement Solutions