unit FreeForm;

interface

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

type
  TForm1 = class(TForm)
    Button1: TButton;
    Button2: TButton;
    Button3: TButton;
    procedure Button1Click(Sender: TObject);
    procedure Button2Click(Sender: TObject);
    procedure Button3Click(Sender: TObject);
    procedure FormDestroy(Sender: TObject);
    procedure FormCreate(Sender: TObject);
  public
    destructor Destroy; override;
  private
    Button4: TButton;
  end;

var
  Form1: TForm1;

implementation

{$R *.DFM}

procedure TForm1.Button1Click(Sender: TObject);
var
  B: TButton;
begin
  B := TButton.Create (self);
  B.Free;
  // Fix: B := nil;
  B.Free;
end;

procedure TForm1.Button2Click(Sender: TObject);
var
  B1, B2: TButton;
begin
  // correct code
  B1 := TButton.Create (self);
  B2 := TButton.Create (B1); // B1 is the owner
  B2.Free;
  B1.Free;
end;

procedure TForm1.Button3Click(Sender: TObject);
var
  B1, B2: TButton;
begin
  // wrong code
  B1 := TButton.Create (self);
  B2 := TButton.Create (B1); // B1 is the owner
  B1.Free;
  B2.Free;
end;

procedure TForm1.FormDestroy(Sender: TObject);
begin
  // OK
  Button1.Free;
end;

destructor TForm1.Destroy;
begin
  Button2.Free; // OK
  inherited Destroy;
  Button3.Free; // OK
  Button4.Free; // Error!
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  Button4 := TButton.Create (self);
end;

end.