ObservableCollection w/ .net 4.0

I have a project that was being compiled in .net 3.5, when I converted it to .net 4.0 I receive a compile error saying:

"The base class or interface of 'MyApplication.ClassName'开发者_JS百科 could not be resolved or in invalid."

This class implements ObservableCollection but other than that there is nothing special about it. Has anyone else ran into this issue?

ObservableCollection class was defined in WindowsBase.dll in v3.0 and was moved to System.dll in v4.0. Maybe you should check your project's references.