Thank you for your interest in RadTextBox.
Currently the only option to change the validation state and the validation message is to use the ChangeValidationState method. If you don't want to use any code behind, you can add this method in the Execute method of a command. Then you can use this command as ActionButtonCommand/ClearButtonCommand and set the state on pressing the action/clear button.
I hope this information helps. Let me know if you need additional assistance here.
the Telerik team
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