begin
  ModalForm := TModalForm.Create(Application);//动态创建窗体
  try
    ModalForm.ShowModal;//显示模式窗体
  finally
    ModalForm.Free; //释放窗体实例
    //ShowMessage(BoolToStr(ModalForm = nil));
    ModalForm := nil; //把窗体变量设为nil
    //ShowMessage(BoolToStr(ModalForm = nil));
  end;
end;

begin
  Form1 := TForm1.Create(Application);
  Form1.show;
end;

begin
  if not Assigned(ModalLessForm) then //确保只创建一个窗口
  begin
    ModalLessForm := TModalLessForm.Create(Application);
  end;
  ModalLessForm.Show;
end;

procedure TModalLessForm.FormClose(Sender: TObject;
  var Action: TCloseAction);
begin
  action := caFree; //当关闭窗口时释放窗体
end;
 
procedure TModalLessForm.FomDestroy(Sender: TObject);
begin
  ModalLessForm := nil; //在窗口销毁时，把modalLessForm变量设为nil
end;