怎样枚举一个pin上的媒体类型？

int _tmain(int argc, _TCHAR* argv[])

{

	CoInitializeEx(NULL, COINIT_APARTMENTTHREADED);

	CLSID clsid;

	//安装ac3filter之后可以看到clsid

	::CLSIDFromString(_T("{A753A1EC-973E-4718-AF8E-A3F554D45C44}"), &clsid);

	IBaseFilter* pFilter = NULL; 

	HRESULT hr = CoCreateInstance(clsid, NULL, CLSCTX_INPROC_SERVER,__uuidof(IBaseFilter), (void **)&pFilter);

	IEnumPins* pEnumPins = NULL;

	hr = pFilter->EnumPins(&pEnumPins);

	if (SUCCEEDED(hr))

	{

		IPin* pPin = NULL;

		while (pEnumPins->Next(1, &pPin, NULL) == S_OK)

		{

			PIN_DIRECTION pinDir;

			//pin的direction可以成功获取，ac3filter上有2个pin，一个in，一个out

			pPin->QueryDirection(&pinDir);

			IEnumMediaTypes* pEnumMediaTypes = NULL;

			HRESULT hr = pPin->EnumMediaTypes(&pEnumMediaTypes);

			if (SUCCEEDED(hr))

			{

				AM_MEDIA_TYPE* pMediaType = NULL;

				//试图枚举类型的时候，hr总是返回S_FALSE，pMediaType依然指向NULL

				hr = pEnumMediaTypes->Next(1, &pMediaType, NULL);

				pEnumMediaTypes->Release();

			}

			pPin->Release();

		}

		pEnumPins->Release();

	}

	return 0;

}