I have an in-house proprietary library developed in .NET 2.0 using VS 2008 . I upgraded to VS 2010 and using the library in a new project based on 4.0. I have added the library as reference but when I am trying to compile I get an error stating that namespace doesn't exists. In addition I tried to change the framework to 2.0 for my new project in VS 2010 . Doing this offloads my project.