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; ...