Firstly, I would recommend you to run through this article for a reference. Depending on the validation that you do, you can easily set ErrorMessage attribute.
Considering the validation of a numeric value when a string one is entered, this is handled by the Framework, not by RadDataForm - that is why the ValidatingItem event is not fired. Please run through this and this articles for a reference.
As for your last requirement, you can try implementing some custom logic in TextChanged event of the TextBox:
this.AddHandler(TextBox.TextChangedEvent, new TextChangedEventHandler(OnTextChanged), true);
private void OnTextChanged(object sender, TextChangedEventArgs e)
Another possible approach would be to implement the logic inside the setter of the property.
the Telerik team
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