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);

        }