You have three opportunities:
1. To create a UserControl and change the root tag to RadWindow, go to code-behind and change the class to inherit from RadWindow instead of UserControl (the approach you described). If this doesn't work, could you please send us a sample project - this will let us investigate what the problem is?
2. To create a UserControl that will represent the content of the Window and when you need to show the window you could create a window, create an instance of your UserControl, add it as a content of the window and call its show method.
3. We don't recommend this, but it is still possible - you could create your Window in any XAML file and call its Show or ShowDialog method. In this case you should be careful with the Name property you set to the Window control, because all shown windows share the same name scope.
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