void EditClient()
 {
   int id = 0;
   string login = "";
   try
   {
     login = lstClients.SelectedItem.ToString();
   }
   catch { return; }
   var clients = from U in db.User
                 where U.Login == login
                 select U;
   foreach (User U in clients)
     id = U.ID_user;
   frmClient frmC = new frmClient(connection, id);
   frmC.ShowDialog();
   db = new DB_CEGSA(connection);
   ReadListClients();
 }
 private void tbtnAdd_Click(object sender, EventArgs e)
 {
   frmClient frmC = new frmClient(connection);
   frmC.ShowDialog();
   ReadListClients();
 }