OpenDialog1.Execute(Self.Handle)

  TCommonDialog = class(TComponent)
    ..........
    function Execute: Boolean; overload; virtual;
    function Execute(ParentWnd: HWND): Boolean; overload; virtual; abstract;
   ...............
  end;

function TCommonDialog.Execute: Boolean;
var
  ParentWnd: HWND;
begin
  if Application.ModalPopupMode <> pmNone then
  begin
    ParentWnd := Application.ActiveFormHandle;
    if ParentWnd = 0 then
      ParentWnd := ApplicationMainHandle;
  end
  else
    ParentWnd := ApplicationMainHandle;
  Result := Execute(ParentWnd);
end;