Fix: The tag ‘TimeUpDown’ does not exist in XML namespace (Silverlight Toolkit)
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 x:Key="StartTimeCellTemplate"> <TextBlock Margin="4" Text="{Binding Begin}" /> </DataTemplate> <DataTemplate x:Key="StartTimeCellEditTemplate"> <!-- <TextBox Margin="4" Text="{Binding Begin, Mode=TwoWay, ValidatesOnExceptions=True, NotifyOnValidationError=true}" /> --> <input:TimeUpDown Format="hh:mm:ss" Value="{Binding Begin, Mode=TwoWay, ValidatesOnExceptions=True, NotifyOnValidationError=true}" /> </DataTemplate> … </ResourceDictionary> [...]