Answer
These classic FND client DLLs are built for the older .NET Framework and, on IFS9-era releases, don't have a .NET Core-compatible equivalent — that direct DLL-based access pattern is essentially frozen at .NET Framework. For an IFS9 environment needing to modernize the integration, the two realistic alternatives are building a custom BizAPI and invoking it (rather than calling the FND DLLs directly), or calling the PL/SQL gateway directly with your own code — both are more work and put more of the integration contract in your own hands. On IFS Applications 10 / IFS Cloud, this whole limitation is moot: Aurena's declarative, REST/OData-based API model replaces the DLL-based access pattern entirely, and is the recommended direction going forward rather than working around the older client library's platform limitation.