u_combo.pas

Here s a demonstration about Auto Drop Combobox

unit U_COMBO;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;

type
  TForm1 = class(TForm)
    Edit1: TEdit;
    Edit2: TEdit;
    ComboBox1: TComboBox;
    Edit3: TEdit;
    procedure ComboBox1Enter(Sender: TObject);
    procedure FormCreate(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}


procedure TForm1.ComboBox1Enter(Sender: TObject);
begin
  IF  LENGTH  ((SENDER AS TCOMBOBOX).TEXT) = 0 THEN
  SENDMESSAGE ((SENDER AS TCOMBOBOX).Handle,CB_SHOWDROPDOWN,1,0);

  // O PARAMETRO WPARAM E 1 : DROPDOWN

end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  FORM1.LEFT :=300;
  FORM1.TOP  :=100;
end;

end.