public class TextBoxHelper
    {
        #region PasteCommand Property

        public static readonly DependencyProperty PasteCommandProperty =
            DependencyProperty.RegisterAttached(
                "PasteCommand",
                typeof(ICommand),
                typeof(TextBoxHelper),
                new FrameworkPropertyMetadata(PasteCommandChanged)
            );

        public static ICommand GetPasteCommand(DependencyObject target)
        {
            return (ICommand)target.GetValue(PasteCommandProperty);
        }

        public static void SetPasteCommand(DependencyObject target, ICommand value)
        {
            target.SetValue(PasteCommandProperty, value);
        }

        static void PasteCommandChanged(DependencyObject sender, DependencyPropertyChangedEventArgs e)
        {
            var textBox = (TextBox)sender;
            var newValue = (ICommand)e.NewValue;

            if (newValue != null)
                textBox.AddHandler(CommandManager.ExecutedEvent, new RoutedEventHandler(CommandExecuted), true);
            else
                textBox.RemoveHandler(CommandManager.ExecutedEvent, new RoutedEventHandler(CommandExecuted));

        }

        static void CommandExecuted(object sender, RoutedEventArgs e)
        {
            var textBox = (TextBox)sender;
            var command = GetPasteCommand(textBox);

            if (command.CanExecute(null))
                command.Execute(null);
        }

        #endregion
    }