public T GetChildOfType<T>(DependencyObject depObj) where T : DependencyObject
        {
            if (depObj == null) return null;

            for (int i = 0; i < VisualTreeHelper.GetChildrenCount(depObj); i++)
            {
                var child = VisualTreeHelper.GetChild(depObj, i);

                var result = (child as T) ?? GetChildOfType<T>(child);
                if (result != null) return result;
            }
            return null;
        }

        void DatePicker_Loaded(object sender, RoutedEventArgs e)
        {
            var dp = sender as DatePicker;
            if (dp == null) return;

            var tb = GetChildOfType<DatePickerTextBox>(dp);
            if (tb == null) return;

            var wm = tb.Template.FindName("PART_Watermark", tb) as ContentControl;
            if (wm == null) return;

            wm.Content = "选择日期";
        }