Copying here my answer at: http://social.msdn.microsoft.com/Forums/en-US/wpf/thread/11a42336-8d87-4656-91a3-275413d3cc19 What seems to work for me is the following (copying from the source code of http://ClipFlair.codeplex.com [currently under development]) note I’m using Build Action = "Page" and Custom ...

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; ...