I finally managed to make VB6 work with the Data Port Component (or the other way around). Why VB6? Although Microsoft killed it, a lot of people out there is still using it and will continue to do so in the foreseeable future. Interestingly, I recieved a lot of requests to develop a Component wrapper in VB6 from the UK. Being an almost complete ignorant, I shied away from this until one of my customers actually started the project.
At first it seemed quite straightforward. First, I had to change all __cdecl exported functions to __stdcall. The need for the second change was very hard to detect. The supporting DLL was designed to either be P/Invoked from .NET or to be directly consumed by a C++ application. This meant shielding all database-related functions with COM initialization code. This also meant exit exceptions when the DLL was consumed by VB6, either on the IDE or on the standalone runtime. After discovering this, there was the threading issue. Apparently one cannot safely call back into VB6 from an external thread, something that is safe in C++ and even on the .NET framework (that is how the C# sample works). The final solution is to start a timer to poll the import or export progress data. A kludge, but it works.
The nice thing about the whole process is that I had to instrument the whole code just to make sure there were no memory leaks or invalid pointers. A typical case of spring cleaning...
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