If you use TimeUpDown control from Silverlight Toolkit in your XAML like below (copy pasting from CaptionGrid at ClipFlair source): <ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" > <DataTemplate ...

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 [...]