SelectAllTextBoxBehavior

   public static class SelectAllTextBoxBehavior
    {
         public static readonly DependencyProperty IsEnabledProperty =
            DependencyProperty.RegisterAttached(
                "IsEnabled",
                typeof(bool),
                typeof(SelectAllTextBoxBehavior),
                new UIPropertyMetadata(false, OnIsEnabledChanged));

        public static bool GetIsEnabled(DependencyObject dependencyObject)
        {
            return (bool)dependencyObject.GetValue(IsEnabledProperty);
        }

        public static void SetIsEnabled(DependencyObject dependencyObject, object value)
        {
            dependencyObject.SetValue(IsEnabledProperty, value);
        }

        private static void OnIsEnabledChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            var textBox = d as TextBox;
            if (textBox == null) return;

            if ((bool) e.NewValue == true)
            {
                textBox.GotFocus += TextBoxOnGotFocus;
            }
            else
            {
                textBox.GotFocus -= TextBoxOnGotFocus;
            }
        }

        private static void TextBoxOnGotFocus(object sender, RoutedEventArgs routedEventArgs)
        {
            var senderBox = sender as TextBox;
            if (senderBox == null) return;

            // select all if text property contain text
            if (senderBox.Text.Length > 0)
            {
                senderBox.SelectAll();
            }
        }
    }