Ah, I see, sorry for the misunderstanding.
Unfortunately, Unity uses a custom compilation pipeline and I don’t think this behaviour can be changed. There’s work towards switching Unity from Mono to the .Net runtime as well as changing the compilation pipeline to a more standard MSBuild-based one – but those changes are probably still a few major Unity releases away ( see this long thread for details / progress ).
In the meantime, you have to work around how it currently works. The metadata you get from the DLL references isn’t enough? From what I understand, you should be able to find the derived types and get the return type of one of their methods (e.g. see this SO answer)?