[lmikle]

Да, забыл, приведение типов надо (as TFileItemData).
А вообще-то все работает. Т.е. ты в своем коде что-то делаешь не так:

Код:

unit Unit1;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls;

type
  TForm1 = class(TForm)
    ListBox1: TListBox;
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
    procedure ListBox1DblClick(Sender: TObject);
    procedure FormDestroy(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

type
  TFileItemData = class
    FileName : String;
    Atrib : String;
    Size : Integer;
    id : Integer;
    rews:Integer;
    function toString : String;
  end;

function TFileItemData.toString : String;
begin
  Result := Format('id: %d; File: %s; Size: %d; Reviews: %d; Attribute: %s',[id,FileName,Size,rews,Atrib]);
end;


procedure TForm1.Button1Click(Sender: TObject);
var
  I : Integer;
  obj : TFileItemData;
begin
  for I := 1 to 5 do
    begin
      obj := TFileItemData.Create;
      obj.FileName := Format('filename-%d',[i]);
      obj.Atrib := 'R';
      obj.Size := Random(1000000);
      obj.id := i;
      obj.rews := i;
      ListBox1.Items.AddObject(obj.toString,obj)
    end;
end;

procedure TForm1.FormDestroy(Sender: TObject);
var
  I : Integer;
  obj : TFileItemData;
begin
  for I := 0 to ListBox1.Items.Count-1 do
    begin
      obj := ListBox1.Items.Objects[i] As TFileItemData;
      obj.Free;
    end;
end;

procedure TForm1.ListBox1DblClick(Sender: TObject);
var
  obj : TFileItemData;
begin
  if ListBox1.ItemIndex > -1 then
    begin
      obj := ListBox1.Items.Objects[ListBox1.ItemIndex] As TFileItemData;
      ShowMessage(obj.toString);
    end;
end;

end.