Like any other WPF control. The validation depends on the framework for WPF so we don't do anything in our controls. Check the attached project. It defines a Binding that has ValidationRule attached. The rule is invalid if the RadNumericUpDown's Value is negative. The Binding also defines the NotifyOnValidationError="True" and ValidatesOnDataErrors="True" so that the red rectangle may appear.
Regards,
Pana
the Telerik team
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