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;