Visual Studio 2010: Reference Assemblies Targeting Higher Framework Version
Visual Studio 2008 did let you reference an assembly A from an Assembly B when A was targeting .NET 3.5 and B was targeting .NET 2.0. Visual Studio 2010 doesn't allow for this any more. The full issue is described on MSDN : You can create applications that reference projects or assemblies that target different versions of the .NET Framework. For example, if you create an application that targets the .NET Framework 4 Client Profile, that project can reference an assembly that targets .NET Framework version 2.0. However, if you create a project that targets an earlier version of the .NET