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?

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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.