I just had some big trouble troubleshooting the Visual Studio / XAML compiler error message: The tag XXX does not exist in XML namespace ‘clr-namespace:YYY’ It turned out this occurred because I had a Silverlight library with Assembly name X and default namespace X (at properties pane), but was using it from a demo project [...]

Just came across http://wangmo.wordpress.com/2007/09/27/themesgenericxaml/ which gave me a hint on why a WPF control wasn’t getting instantiated correctly when loaded from an external assembly (dll): to load generic.xaml for WPF, at the start of Properties\AssemblyInfo.cs you need (note this isn’t used/needed in Silverlight): using System.Windows; ...