// это в дизайнере
HdrbindingSource.DataSource = typeof(WindowsFormsApplication1.mdm_Dogovor);

            this.textBox1.DataBindings.Add(new System.Windows.Forms.Binding("Text", this.HdrbindingSource, "num", true));

     db = new DataDogovorDataContext(DBConnection);
    HdrbindingSource.DataSource = db.Dogovors;

            var dog = from c in db.Dogovors
                      where c.dg_id == id //id -документа
                      select c; 

            if (dog == null)
            {
                HdrbindingSource.DataSource = db.Dogovors;
                HdrbindingSource.AddNew();

            HdrbindingSource.EndEdit();
            this.Validate();
            try
            {
                db.SubmitChanges();
            }
            catch (Exception err)
            {

                MessageBox.Show(err.Message);
            }