.net - 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

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>