My WPF Application cannot locate resource 'Mainwindow.xaml'
In my case I needed to use pack URI syntax to set SrartUpUri property of my App.xaml file to point to a new location of my MainWindow.xaml, as so:
<Application x:Class="TrafficLights.Controller.App" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" StartupUri="pack://application:,,,/View/MainWindow.xaml">More on pack URIs here:
http://msdn.microsoft.com/en-us/library/aa970069(v=vs.110).aspx
Hope it helps. Good luck!