A few years we did just this: Create a .NET Core project, but referencing the full .NET Framework because we reference the Act! framework.
This worked fine.
Now we want to do it again for something the API does not support (yet), with our beloved (ehrm) V21, but the probleme
I've attached a zip file with a very small VS solution so you can verify it. The project 'WhyProductMismatchExceptoin" has the default 'ValuesController', and in the GET endpoint it instantiates an Act! framework with a hardcoded database and credentials, and should write a JSON representation of the Myrecord to the output. It doesn't, it crashes with a ProductVersionMismatch.
That's strange, because the DLL's are of the same version of the Act! database.
Even stranger, i've added a second project in the solution 'AndWhyDoesItWorkHere', with the exact same logon-and-myrecord-JSON code in it, which it writes to the console. Works perfectly. Why?