I create a wpf project only contains a TextBox control. And The TextBox Binds to a ValidationRule .The rule is always return false to test the error template.
1) if The window base class is Window , and call ShowDialog() method. When I input some chars into TextBox, the TextBox 's show the red border, it is ok.
2) if I change the base class from Window to UserControl, insert the usercontrol to another wpf window element, and call Dispath.Run() method in usercontrol. The ValidationRule work correctly and always return false, but the TextBox's default error template did no show red border.