C# using statement rejected in one class but accepted in another in same project

I have a WPF desktop project targeting .Net 4.5.1. Two identical using statements (verified by cut and paste):

using System.Windows.Data;

using System.Windows.Controls;

are accepted in one class but rejected in another. The type or namespace 'Data' does not exist in namespace 'System.Windows'

• Both classes are in the same project.
• Both classes are in the same namespace.
• They are in different folders within the project structure.
• I have project references to both PresentationCore and PresentationFramework.
• The project is targeting Mixed Platforms/ Any CPU