public static class WindowEventsBinding
    {
        #region Closing

        /// <summary>
        /// Closing Event.
        /// </summary>
        public static readonly DependencyProperty ClosingProperty = DependencyProperty.RegisterAttached( "Closing", typeof( ICommand ), typeof( WindowEventsBinding ), new UIPropertyMetadata( new PropertyChangedCallback( WindowEventsBinding.ClosingChanged ) ) );

        public static ICommand GetClosing(DependencyObject obj)
        {
            return obj.GetValue( WindowEventsBinding.ClosingProperty ) as ICommand;
        }

        public static void SetClosing(DependencyObject obj, ICommand value)
        {
            obj.SetValue( WindowEventsBinding.ClosingProperty, value );
        }

        private static void ClosingChanged( DependencyObject target, DependencyPropertyChangedEventArgs e )
        {
            Window window = target as Window;
            if (window == null) return;

            if (e.NewValue != null) window.Closing += WindowEventsBinding.Window_Closing;
            else window.Closing -= WindowEventsBinding.Window_Closing;
        }

        private static void Window_Closing( object sender, global::System.ComponentModel.CancelEventArgs e )
        {
            Window window = sender as Window;
            if (window == null) return;

            ICommand closingBinding = window.GetValue( WindowEventsBinding.ClosingProperty ) as ICommand;
            if (closingBinding == null) return;

            if (closingBinding.CanExecute( null )) closingBinding.Execute( null );
            else e.Cancel = true;
        }

        #endregion
    }

public static class WindowEventsBinding
    {
        // ...
        private static void ClosingChanged( DependencyObject target, DependencyPropertyChangedEventArgs e )
        {
            Window window = target as Window;
            if (window == null) return;

            if (e.OldValue == null && e.NewValue != null) window.Closing += WindowEventsBinding.Window_Closing;
            
            if (e.NewValue == null && e.OldValue != null) window.Closing -= WindowEventsBinding.Window_Closing;
        }
    }