public class FocusSelectTextBox : TextBox
    {
        public FocusSelectTextBox()
        {
            AddHandler(PreviewMouseLeftButtonDownEvent, new MouseButtonEventHandler(SelectivelyIgnoreMouseButton), true);
            AddHandler(GotKeyboardFocusEvent, new RoutedEventHandler(SelectAllText), true);
            AddHandler(MouseDoubleClickEvent, new RoutedEventHandler(SelectAllText), true);
        }

        private static void SelectivelyIgnoreMouseButton(object sender, MouseButtonEventArgs e)
        {
            // Find the TextBox
            DependencyObject parent = e.OriginalSource as UIElement;
            while (parent != null && !(parent is TextBox))
                parent = VisualTreeHelper.GetParent(parent);

            if (parent == null) return;
            var textBox = (TextBox)parent;
            if (textBox.IsKeyboardFocusWithin) return;
            // If the text box is not yet focussed, give it the focus and
            // stop further processing of this click event.
            textBox.Focus();
            e.Handled = true;
        }

        private static void SelectAllText(object sender, RoutedEventArgs e)
        {
            var textBox = e.OriginalSource as TextBox;
            if (textBox != null)
                textBox.SelectAll();
        }
    }

http://stackoverflow.com/questions/660554/how-to-automatically-select-all-text-on-focus-in-wpf-textbox/661224#661224

ClickSelectTextBox

C#