AddCalculation_EventHandler.cs

public static readonly DependencyProperty AProperty =
    DependencyProperty.Register(nameof(A),
                                typeof(int),
                                typeof(AddCalculation),
                                new PropertyMetadata(0, AddNumbers));
private static void AddNumbers(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
    var ac = (AddCalculation)d;
    ac.SetValue(ResultPropertyKey, ac.A + ac.B);
}