unit TestClass;

interface

type
  TTest = class
  protected
    ProtectedData: Integer;
  public
    PublicData: Integer;
    function GetValue: string;
  end;

implementation

uses
  SysUtils;

function TTest.GetValue: string;
begin
  Result := Format ('Public: %d, Protected: %d',
    [PublicData, ProtectedData]);
end;

end.

unit ProtForm;

interface

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

type
  TForm1 = class(TForm)
    Button1: TButton;
    Button2: TButton;
    procedure Button1Click(Sender: TObject);
    procedure Button2Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.DFM}

uses
  TestClass;

procedure TForm1.Button1Click(Sender: TObject);
var
  Obj: TTest;
begin
  Obj := TTest.Create;
  Obj.PublicData := 10;
//  Obj.ProtectedData := 20;  //won't compile
  ShowMessage (Obj.GetValue);
  Obj.Free;
end;

type
  TFake = class (TTest);

procedure TForm1.Button2Click(Sender: TObject);
var
  Obj: TTest;
begin
  Obj := TTest.Create;
  Obj.PublicData := 10;
  TFake (Obj).ProtectedData := 20; // compiles!
  ShowMessage (Obj.GetValue);
  Obj.Free;
end;

end.