Referencing dlls built with older .Net versions

In a project using .Net4 - should there be any issues referencing dlls built for older versions of .Net? We're talking 3rd Party dlls here.

I'm asking this as a general question. Specifically I have a problem referencing FluentNHibernate.dll - built with .Net3.5. It worke开发者_开发百科d justed fine before I updated my project from .Net3.5 to .Net4.

No in general you can have a .Net 4.0 project reference any dlls even if they were compiled against older versions of .Net.

Looking at your linked question: It could be that libraries that inject/modify IL are affected by versioning problems because the injections are not under the control of the .Net runtime system.