如何使用C＃将空值返回到数据库

Question

我创建了一个函数，如果字符串的字段长度小于2个字符，该函数应该将空值发送回我的数据库，但是，它将在数据库上将其更改为00-00-00。有人能指出我正确的方向吗？public string dateChanger(string txtBox)          if (txtBox.Length < 2)            {                return DBNull.Value.ToString();            }            else            {                var date = txtBox;                var name = date.Substring(6, 4) + "-" + date.Substring(3, 2) + "-" + date.Substring(0, 2);                return name;            }}下面是我用来调用函数“ dateChanger”并插入数据库的代码     [HttpPost]        public JsonResult AjaxMethodSaveStudent(string cboStudentName = "", string cboStudentSurname = "", string txtStudentMiddleName = "", string txtStudentNumber = "", string txtStudentDOB = "", string txtStudentPreferredName = "", string txtStudentPropertyName = "", string txtStudentRegImmiCardNumber = "")        {            long studentId = 0;            string constr = ConfigurationManager.ConnectionStrings["ConString"].ConnectionString;            using (MySqlConnection con = new MySqlConnection(constr))            {                string queryStudent = "INSERT INTO `STUDENT REGISTER` " +                    "(`first name`, " +                    "`surname`, " +                    "`Middle Names`," +                    "`DATE OF BIRTH`," +                    "`ImmiCardNumber`) " +                    "VALUES" +                    "('" + cboStudentName + "'," +"'" + cboStudentSurname + "'," +"'" + txtStudentMiddleName + "'," +"'" + dateChanger(txtStudentDOB) + "'," +"'" + txtStudentRegImmiCardNumber + "') ";                using (MySqlCommand cmd = new MySqlCommand(queryStudent))                {                    cmd.Connection = con;                    con.Open();                    cmd.ExecuteNonQuery();                    studentId = cmd.LastInsertedId;                    con.Close();                }            }            return Json(new { success = true, studentId = studentId }, JsonRequestBehavior.AllowGet);        }