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Via a friend of mine, so don't blame me. I just ported it to LabVIEW .

Whooo Hooo, it came through!! I'm really happy to be at this point, its a new direction for me but one that will be a lot of fun, and bloody hard work. Thanks to the guy's at NI Australia for your support

LabVIEW 2009 introduces a "friends" relationship. Essentially ClassA can declare ClassB (or another type of library, or even a VI) to be a friend. Then ClassB can call any methods ClassA sets as "community" scope. NI has updated these pages with the details: LabVIEW Object-Oriented Programming: The Decisions Behind the Design Creating LabVIEW Classes

How about DROOP?

Why not turn the whole idea around? You do want callback support and ActiveX (long ago) and .Net (since version 8) interfaces in LabVIEW support it out of the box. So the real solution would be to write a thin ActiveX or .Net wrapper around your DLL code that translates the DLL callback in an according ActiveX or .Net event. Then your DLL invokes the callback funciton which in turn sends the ActiveX/.Net event to LabVIEW and LabVIEW invokes the according callback VI which then returns whatever data is required to the ActiveX/.Net event and that returns control to the callback. Yes it is not trivial and it would not work for tight low level kernel type callback drivers, but nothing will directly work with such callback drivers from a high level environment like LabVIEW. You could make it sort of work from a simple C program but certainly not a .Net application or such either. If you need to pass large amounts of data from the callback to LabVIEW or vice versa you would have to opt for an in-process ActiveX or .Net wrapper otherwise you can go with an out of process wrapper too. ActiveX/.Net will take care of marshaling the data for out of process servers between said server and LabVIEW (and .Net may do marshaling no matter what). Marshaling is ok for small amounts of data but if you plan to move large amounts of data it is going to create an additional bottleneck. Rolf Kalbermatter