關於在COM中使用可選參數的研究

在Delphi6中設計COM組件方法時，在 Parameter flags dialog 中，選擇Optional選項，即輸入參數為可選，現就給出如何實現可選參數實例。  監測函數 function VarIsEmptyParam(const V: Variant): Boolean; begin   Result := (TVarData(V).VType = varError) and             (TVarData(V).VError = $80020004); {DISP_E_PARAMNOTFOUND} end; 實例 uses ComServ, Variants, Dialogs, SysUtils; procedure TOptionalParamsDemo.GetParams(const Param1: WideString; Param2,Param3: OleVariant; Param4: Integer); var   s2, s3: WideString; begin   if VarIsEmptyParam(Param2) then     s2 := ' empty '   else     S2 := ' not empty ';   if VarIsEmptyParam(Param3) then     s3 := ' empty '   else     S3 := ' not empty ';   ShowMessage(Format('Param1[%s] Param2[%s] Param3[%s] Param4[%d]',[Param1, s2, s3, Param4])); end;

　　  procedure TForm1.Button1Click(Sender: TObject); var   V: Variant; begin   V := IDispatch(CoOptionalParamsDemo.Create());   V.GetParams('123'); end;   procedure TForm1.Button2Click(Sender: TObject); var   AIntf: IOptionalParamsDemo;   V: Variant; begin   AIntf := CoOptionalParamsDemo.Create();  // AIntf.GetParams('', EmptyParam, 9, 4);   V := IDispatch(AIntf);  // V.GetParams('123', '123', '123', '123');   V.GetParams(); end;   procedure TForm1.Button3Click(Sender: TObject); var   AIntf: IOptionalParamsDemo;   V: Variant; begin   AIntf := CoOptionalParamsDemo.Create();  // AIntf.GetParams('', EmptyParam, 9, 4);   V := IDispatch(AIntf);   V.GetParams('123', '123');  // V.GetParams(); end;   procedure TForm1.Button4Click(Sender: TObject); var   V: Variant; begin   V := IDispatch(CoOptionalParamsDemo.Create());   V.GetParams('123', '123', 123); end; procedure TForm1.Button5Click(Sender: TObject); var   V: Variant; begin   V := IDispatch(CoOptionalParamsDemo.Create());   V.GetParams('123', 123, 123, 6); end;