Can build console app with mongocxx but not DLL project VS2019

Hi all,

i used microsofts vcpkg to compile and install mongo-cxx-driver_3.4.0-5 . It works correctly from a console app in visual studio 2019 on windows 10, but if i try and apply the same includes / usings in a DLL project i get errors such as :

invalid type for a bit field
expected a ')'

Have a missed something basic here?

Many Thanks