Assembly: System.Windows (in System.Windows.dll)
The method is called before any event handler for the KeyDown event is called. This method allows derived classes to handle the KeyDown event without attaching a delegate. This is the preferred technique for handling the event in a derived class.
The TextBox control handles navigation and edit keys when it performs an action, but it does not handle keys related to text input. Derived classes can block text input by handling keys related to text input.Notes to Inheritors
If you override the method, you can prevent registered delegates from receiving events by setting Handled to true. For more information about routed events and the Handled property, see Events for Windows Phone 8.