thanks a lot, Kristofer Gafvert.

I implemented it in C++ as follow:

/////////////////////////  CODE START  //////////////////////////
BOOL GetDefaultDoc()
{
   IADsContainer* iContainer = NULL;
   IADs* iAds = NULL;

   HRESULT hr = ADsGetObject(L"IIS://localhost/w3svc",
       IID_IADsContainer, (void**)&iContainer);
   if (SUCCEEDED(hr)) {
       hr = iContainer->GetObject(_bstr_t("IIsWebServer"),
           _bstr_t("1"), (IDispatch**)&iAds);
       if (SUCCEEDED(hr)) {
           VARIANT buf = { 0 };
           if (S_OK == iAds->Get(_bstr_t("DefaultDoc"), &buf)) {
               LogMessageW(V_BSTR(&buf)); // Log the default name
               return TRUE;
           }
       }
   }

   return FALSE;
}
/////////////////////////  CODE END  //////////////////////////

But I have another question:
we can config the default webpage's name at least 3 places in IIS: the
first is the topmost, it contains all the websites; the second is
website in it; and the third is the virtual directory of a website.

how to get the default webpage's name in those three places?

in another words, how to get those names given a url such as
"http://localhost/DirectoryName/"?

Best Regards