Gets or sets the associated with this RoutedEventArgs instance.
Assembly: PresentationCore (in PresentationCore.dll)
Attempted to change the value while the event is being routed.
You cannot set this value on a RoutedEventArgs that has already been routed (for instance, if you obtained the arguments through a handler). Attempting to do so will generate an exception. You can only set it on an instance that has not yet been used to generate an invocation of the event.
The value of cannot be nullptr at any time.