se = new skladEntities(new Uri(Properties.Settings.Default.ServiceString));
            firm_localBindingSource.DataSource = from ff in se.firm
                                                 where ff is firm_local 
                                                 select ff;

  FirmLocalDetail fld = new FirmLocalDetail();
  fld.firm_localBindingSource.DataSource = this.firm_localBindingSource.DataSource;
  frm_local.BeginEdit();

  if (fld.ShowDialog() == System.Windows.Forms.DialogResult.Cancel)
                {
                    frm_local.CancelEdit();
                }

    [Serializable]
    public abstract partial class firm
    {
        private byte[] originalState;
        private firm firmItem;

        public virtual void BeginEdit()
        {
            BinaryFormatter formatter = new BinaryFormatter();
            firmItem = this;

            using (MemoryStream stream = new MemoryStream())
            {
                formatter.Serialize(stream, this);
                stream.Position = 0;
                originalState = new byte[stream.Length];
                // сохраняемся
                stream.Read(originalState, 0, (int)stream.Length);
            }
        }

        public virtual void CancelEdit()
        {
            if (originalState != null)
            {
                BinaryFormatter formatter = new BinaryFormatter();

                using (MemoryStream stream = new MemoryStream())
                {
                    stream.Write(originalState, 0, originalState.Length);
                    stream.Position = 0;
                    firmItem = this;
                    // восстанавливаемся
                    firmItem = (firm)formatter.Deserialize(stream);
                    originalState = null;
                }
            }
      }

// восстанавливаю объект из буфера   
frm_local.CancelEdit();
// frm_local после этого смотрит на восстановленный объет
frm_local = (firm_local) frm_local.firmItem;


//а здесь не идет, пишет  
// The left-hand side of an assignment must be a variable, property or indexer

(firm_localDataGridView.CurrentRow.DataBoundItem as firm_local) =
                        frm_local;