Why does adding a reference to project targeting .NET Framework 4.0 fail?

Question

We have two projects that are both class libraries. Project 1 is a VS 2008 project and targets the .NET Framework 3.5. Project 2 is a VS 2010 (release candidate) project that targets the .NET Framework 4.0. When I try to add a reference to Project 2 in Project 1, it fails with a less than informative error message. I know that if I change the target Framework for Project 2 to 3.5, then adding the reference will work. My question is, if I don't change the target frameworks, but convert Project 1 to VS 2010, will the referencing work? Stated another way, is there some inherent incompatiblity between class libraries targeting different framework versions, or is it failing for me because VS 2008 doesn't know about the 4.0 framework?

Answer 1

Yes it is a problem. Much like you can't add a reference to a .NET 2.0 assembly from a .NET 1.0 or 1.1 project. The .NET 4.0 is a new runtime version of the framework (CLR). The .NET 3.5 and 3.0 releases both use the 2.0 runtime version of the framework (CLR).

You can always add a reference from an assembly built with a newer runtime to an assembly with an older runtime, but not the other way around.

From .NET 2.0 (3.0 and 3.5):
* Can reference .NET 2.0 (3.0 and 3.5) assemblies
* Can NOT reference .NET 4.0 assemblies

From .NET 4.0:
* Can reference .NET 4.0 assemblies
* Can reference .NET 2.0 (3.0 and 3.5) assemblies

Answer 2

Framework versions 2, 3.0, 3.5 run on CLR version 2, while framework version 4 runs on CLR version 4: the version 2 CLR would not load an assembly that states it requires CLR 4, so Visual Studio is trying (badly, it sounds like) to tell you that. So, to answer your question, yes, if you update your project 1 to VS 2010 and change the target framework to version 4, (I think it does this by default), it will be able to reference project 2.