I struggled with an error in my XAML on a new project this weekend. The error is a design time error. That is, my project compiled and ran perfectly. But, the designer had the error: The name ‘RegionManager’ does not exist in the namespace “http://www.codeplex.com/pr... This occurred in Visual Studio 2012 with the Prism 4.1 libraries that were especially compiled against .NET Framework 4.5. And that annoyingly had to be modified and resigned for the appropriate version of the Unity 3 library. It ......