Text长度小于7时，边框变为红色

public class L2BConverter : IValueConverter    {        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)        {            int textLength = (int)value;            return textLength > 6 ? true : false;        }        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)        {            throw new NotImplementedException();        }    }