While working on a new Windows WPF application today, I ran into a serious problem. I had made a UserControl class and wanted to use it in my application. After defining it in the XAML of the main window, Visual Studio said that the entire assembly could not be found. I spent several hours checking the code: namespaces, assemblies, cleaning/rebuilding etc. but nothing changed anything. I eventually found out that Visual Studio has problems when working with projects over mapped network drives. I copied the project from my server onto my desktop and voila! The error was gone.