Советы по Delphi. Версия 1.4.3 от 1.1.2001

 FPickList.Parent := Self;
 FPickList.IntegralHeight := True;
 FPickList.ItemHeight := 11;
 FPickList.Items.CommaText :='1,2,3,4,5,6,7,8,9,0';
end;

destructor TTestDropEdit.Destroy;
begin
 FPickList.Free;
 inherited;
end;

procedure TTestDropEdit.CloseUp(Accept: Boolean);
begin
 if FPickList.Visible then begin
 if GetCapture <> 0 then SendMessage(GetCapture, WM_CANCELMODE, 0, 0);
 SetWindowPos(FPickList.Handle, 0, 0, 0, 0, 0, SWP_NOZORDER or SWP_NOMOVE or SWP_NOSIZE or SWP_NOACTIVATE or SWP_HIDEWINDOW);
 if FPickList.ItemIndex <> -1 then Text := FPickList.Items.Strings[FPickList.ItemIndex];
 FPickList.Visible := False;
 Invalidate;
 end;
end;

procedure TTestDropEdit.DropDown;
var
 P: TPoint;
 I,J,Y: Integer;
begin
 if Assigned(FPickList) and (not FPickList.Visible) then begin
 FPickList.Width := Width;
 FPickList.Color := Color;
 FPickList.Font := Font;
 FPickList.Height := 6 * FPickList.ItemHeight + 4;
 FPickList.ItemIndex := FPickList.Items.IndexOf(Text);
 P := Parent.ClientToScreen(Point(Left, Top));
 Y := P.Y + Height;
 if Y + FPickList.Height > Screen.Height then Y := P.Y - FPickList.Height;
 SetWindowPos(FPickList.Handle, HWND_TOP, P.X, Y, 0, 0, SWP_NOSIZE or SWP_NOACTIVATE or SWP_SHOWWINDOW);
 FPickList.Visible := True;
 Invalidate;
 Windows.SetFocus(Handle);
 end;
end;

procedure TTestDropEdit.CMCancelMode(var Message: TCMCancelMode);
begin
 if (Message.Sender <> Self) and (Message.Sender <> FPickList) then CloseUp(False);
end;

procedure TTestDropEdit.WMKillFocus(var Message: TMessage);
begin
 inherited;
 CloseUp(False);
end;

procedure TTestDropEdit.WndProc(var Message: TMessage);
 procedure DoDropDownKeys(var Key: Word; Shift: TShiftState);
 begin
 case Key of
 VK_UP, VK_DOWN:
 if ssAlt in Shift then begin
 if FPickList.Visible then CloseUp(True)
 else DropDown;
 Key := 0;
 end;
 VK_RETURN, VK_ESCAPE:
 if FPickList.Visible and not (ssAlt in Shift) then begin
 CloseUp(Key = VK_RETURN);
 Key := 0;
 end;
 end;
 end;
begin
 case Message.Msg of
 WM_KeyDown, WM_SysKeyDown, WM_Char:
 with TWMKey(Message) do begin
 DoDropDownKeys(CharCode, KeyDataToShiftState(KeyData));
 if (CharCode <> 0) and FPickList.Visible then begin
 with TMessage(Message) do SendMessage(FPickList.Handle, Msg, WParam, LParam);
 Exit;
 end;
 end
 end;
 inherited;
end;
end.

Программное открытие ComboBox II

Delphi 1

procedureTForm1.ComboBox1Enter(Sender:TObject);
begin
 SendMessage(ComboBox1.Handle, CB_SHOWDROPDOWN, Integer(True), 0);
end;

Поместите эту строку в обработчик события OnEnter ComboBox:

SendMessage(combobox1.Handle, CB_SHOWDROPDOWN, 1, 0);

Измените третий параметр (1) на 0, если вы хотите спрятать список.