Linq更新不工作

我试了很多，但都是徒劳的。
我写了一个LINQ代码，但不能在数据库中保存更改。
它没有给出错误，也没有更新记录。

class Program
{
    [Table(Name = "mainframe_replication")]
    public class mainframe_replication
    {
        private string _REPL_GUID;
        [Column(IsPrimaryKey = true, Storage = "_REPL_GUID")]
        public string REPL_GUID
        {
            get { return this._REPL_GUID; }
            set { this._REPL_GUID = value; }
        }
        private string _REPL_TYPE;
        [Column(Storage = "_REPL_TYPE")]
        public string REPL_TYPE
        {
            get { return this._REPL_TYPE; }
            set { this._REPL_TYPE = value; }
        }
        private string _RPT_ID;
        [Column(Storage = "_RPT_ID")]
        public string RPT_ID
        {
            get { return this._RPT_ID; }
            set { this._RPT_ID = value; }
        }
        private string _RPT_VERS;
        [Column(Storage = "_RPT_VERS")]
        public string RPT_VERS
        {
            get { return this._RPT_VERS; }
            set { this._RPT_VERS = value; }
        }
        private string _RPT_BYTES;
        [Column(Storage = "_RPT_BYTES")]
        public string RPT_BYTES
        {
            get { return this._RPT_BYTES; }
            set { this._RPT_BYTES = value; }
        }
        private string _REPL_DTM;
        [Column(Storage = "_REPL_DTM")]
        public string REPL_DTM
        {
            get { return this._REPL_DTM; }
            set { this._REPL_DTM = value; }
        }
        private string _NOTIF_ID;
        [Column(Storage = "_NOTIF_ID")]
        public string NOTIF_ID
        {
            get { return this._NOTIF_ID; }
            set { this._NOTIF_ID = value; }
        }
    }
    public class MyPoco
    {
        public string ReportId { get; set; }
        public string Reportversion { get; set; }
        public string ReportBytes { get; set; }
        public string ReportDate { get; set; }
        public string NotifId { get; set; }
        public string RecipAdd { get; set; }
    }
    public static string loglocation;
    static void Main(string[] args)
    {
        try
        {
            using (DataClasses1DataContext db = new DataClasses1DataContext())
            {
                Table<NOTIF_RECIP> NOTIF_RECIP_alias = db.GetTable<NOTIF_RECIP>();
                Table<NOTIF_SCHED> NOTIF_SCHED_alias = db.GetTable<NOTIF_SCHED>();
                Table<mainframe_replication> mainframe_replication_alias = db.GetTable<mainframe_replication>();
                var ids = NOTIF_SCHED_alias.Select(x => x.NOTIF_RPT_ID).ToArray();
                foreach (string notif_sched_data in ids)
                {
                    var repljoinmf = mainframe_replication_alias
                        .Join(NOTIF_RECIP_alias, mfr => mfr.RPT_ID, nr => nr.NOTIF_RECIP_ID, (mfr, nr)
                            => new MyPoco { ReportId = mfr.RPT_ID, Reportversion = mfr.RPT_VERS, ReportBytes = mfr.RPT_BYTES.ToString(), ReportDate = mfr.REPL_DTM.ToString(), NotifId = mfr.NOTIF_ID, RecipAdd = nr.NOTIF_RECIP_ADDR });
                    foreach (var repljoinmf_data in repljoinmf)
                    {
                        repljoinmf_data.NotifId = "abc";
                        //DO STUFF 
                        //  repljoinmf_data.NotifId = "Changedxyz";
                    }
                    db.SubmitChanges();
                }
            }
        }
        catch (Exception e)
        {
            Console.WriteLine(e.ToString());
        }
    }
}

在提交更改时没有任何错误。
我需要改变什么?
任何建议都会有帮助的。