數據庫中爲nulll的值，對應dataTable中的值爲dbnull.value

 public void Test()
{
    string sql = "select * from StudentClass";
            DataTable dt = dBHelper.ExecuteReader(sql);
            List<StudentClass> classes = new List<StudentClass>();
            foreach (DataRow dr in dt.Rows)
            {
                StudentClass stuClass = new StudentClass();
                stuClass.ClassId = (int)dr["ClassId"];
                stuClass.ClassName = dr["ClassName"].ToString();
                if (dr["testTime"] != DBNull.Value)
                {
                    stuClass.TestTime = Convert.ToDateTime(dr["testTime"].ToString());
                }
                if (dr["num"] != DBNull.Value)
                {
                    stuClass.num = int.Parse(dr["num"].ToString());
                }
                classes.Add(stuClass);
            }
}
 class StudentClass
    {
        public int ClassId { get; set; }
        public string ClassName { get; set; }

        public DateTime TestTime { get; set; }
        public int num { get; set; }
    }

備註：如果數據庫裏面的值爲null,則dataTable中對應的值爲dbnull.value，如dr["num"],這個值轉成string之後爲空字符串。