关于在COM中使用可选参数的研究
作者：wangzhidong(Steven Bob)
创作日期：2002-3-27
在Delphi6中设计COM组件方法时，在 Parameter flags dialog 中，选择Optional选项，即输入参数为可选，现就给出如何实现可选参数实例。
监测函数
function VarIsEmptyParam(const V: Variant): Boolean;
begin
Result := (TVarData(V).VType = varError) and
(TVarData(V).VError = $80020004); {DISP_E_PARAMNOTFOUND}
end;
实例
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;