Xamarin.Forms Behaviors are created by deriving from the Behavior or Behavior<T> class, where T is the type of the control (Entry, etc) to which the Behavior should apply. Create a new class and which inherits from the Behavior or Behavior<T> class. We need to override OnAttachedTo and OnDetachingFrom method from our validation class. The OnAttachedTo method is fired immediately after the behavior is attached to a control. This can be used to register event handlers or perform other setup that's required to support the behavior functionality. The OnDetachingFrom method is fired when the behavior is removed from the control. This method receives a reference to the control to which it is attached, and is used to perform any required cleanup. Register.xaml <ContentPage.Resources> <ResourceDictionary> <Style x:Key="baseStyle" TargetType="Label"> <Setter Property="XAlign...