procedure TForm1.freebtnClick(Sender: TObject);
begin
//这里已经进行判断了，为什么点击第二次释放按钮的时候，仍然会执行base.free动作？
  if base <> nil then 
    base.Free;
end;

unit Unit1;    interface    uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;    type
  TBase = class(TObject)
  public
    procedure Showmsg; virtual;
    destructor Destroy; override;
  end;
  TDerived = class(TBase)
  public
    procedure Showmsg; override;
    destructor Destroy; override;
  end;
type
  TForm1 = class(TForm)
    freebtn: TButton;
    procedure freebtnClick(Sender: TObject);
    procedure FormCreate(Sender: TObject);
  private
    { Private declarations }
  public
    base: Tbase;
  end;    var
  Form1: TForm1;    implementation    {$R *.dfm}    { TBase }
destructor TBase.Destroy;
begin
  inherited;
  showmessage('Tbase free');
end;    procedure TBase.Showmsg;
begin
  showmessage('hello world! by Tbase');
end;    { TDerived }    destructor TDerived.Destroy;
begin
  inherited;
  showmessage('Tderived free');
end;    procedure TDerived.Showmsg;
begin
 // inherited;
  showmessage('hello world! by TDerived!');
end;    procedure TForm1.freebtnClick(Sender: TObject);
begin
  if base <> nil then
    base.Free;
end;    procedure TForm1.FormCreate(Sender: TObject);
begin
  base := TDerived.Create;
  base.Showmsg
end;    end.     

引言:

procedure TForm1.freebtnClick(Sender: TObject);
begin
//这里已经进行判断了，为什么点击第二次释放按钮的时候，仍然会执行base.free动作？
  if base <> nil then 
    base.Free;
end;

改成如下寫法就OK了,某些物件Free後會自動設為nil,某些又不會,至於那些會那些不會,我沒去研究。
procedure TForm1.freebtnClick(Sender: TObject);
begin
//这里已经进行判断了，为什么点击第二次释放按钮的时候，仍然会执行base.free动作？
  if Assigned(base) then 
    FreeAndNil(base);
end;

引言: aaa.free 後雖然aaa object所佔的記憶體被釋放但 aaa仍指到那塊被釋放的記憶體 所以比較保險的寫法為free之後 aaa := nil; 在使用aaa方法前再加 if Assigned(aaa) then aaa.XXX .... FreeAndNil 同等於 Free之後再assign nil 給該變數

procedure FreeAndNil(var Obj);
var
  Temp: TObject;
begin
  Temp := TObject(Obj);
  Pointer(Obj) := nil;
  Temp.Free;
end;