function GridSelectAll(Grid: TDBGrid): Longint;

begin

  Result := 0;

  Grid.SelectedRows.Clear;

  with Grid.Datasource.DataSet do

  begin

    First;

    DisableControls;

    try

      while not EOF do

      begin

        Grid.SelectedRows.CurrentRowSelected := True;

        inc(Result);

        Next;

      end;

    finally

      EnableControls;

    end;

  end;

end;

procedure TForm1.Button1Click(Sender: TObject);

begin

  GridSelectAll(DBGrid1);

end;