我創建了一個函數，如果字符串的字段長度小于2個字符，該函數應該將空值發送回我的數據庫，但是，它將在數據庫上將其更改為00-00-00。有人能指出我正確的方向嗎？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;            }}下面是我用來調用函數“ dateChanger”并插入數據庫的代碼     [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);        }