Use commercial dll in open source project


I’m planning to create an open-source project that extends functionality of a commercial product. To be able to do this, I need to use a c++ dll from this commercial product. However, the dll does not have headers or a lib file. So it’s not really intended to be used. However, the dll is used in a .NET dll that is part of SDK-libraries of the commercial product. But I prefer a non .NET solution.

Am I allowed to use that dll? I assume that I’m not allowed to distribute it, but that shouldn’t be a problem since users of my project need to have the commercial product.

To be clear, the dll is not part of some plug-in architecture. There exists no plug-in architecture in that product.