This (the source of the files you’ve been working with) will likely help even more:
There’s a C++ example there, I believe.
Seeing how C++ calls the C# library functions may illuminate how to conform the C# file to work with Xojo, or how to conform Xojo to work with it.
Or, a small C++ bridge could be created between Xojo and the C# library that somehow covers passing the various types back and forth. (MemoryBlocks and Pointers would be the most useful…)