Framework compatibility

Question

I have a class library which is built on .NET Framework v4.0. Is it possible to add this as a reference in a console or web application targeted for v3.5 framework?

Answer 1

I would suggest no because they are using completely different CLR. As a rule, backwards compatibility = yes, forwards compatibility = no.

Answer 2

I just created a Framework 3.5 Console application and added a reference to a 4.0 dll I had handy.

The compiler was not happy about this at all.

The (empty) program ran once (console appeared, and then closed) and then 40 warnings arrived in the error list, relating to version problems, and the namespace of the dll could not be accessed at all.

I'm going to agree with Sasquiha and say that this looks like a no-go.

Answer 3

The metadata format has changed in .NET 4.0. Version 2 of the CLR, the one you get when you target any version of .NET between 2.0 and 3.5 does not know how to read it. Same thing happened between 1.1 and 2.0. Version 4 of the CLR knows how to read the old formats without a problem. As long as you have a reference to a non-framework assembly that was compiled to target version 4 then you have to use .NET 4.0.

Retargeting an existing EXE project that doesn't otherwise requires a reference to that assembly is possible. Add a .config file to the project and paste this:

<startup useLegacyV2RuntimeActivationPolicy="true">
   <supportedRuntime version="v4.0"/>
</startup>