用http协议获取下载文件的大小

//获取http服务器上要下载文件的大小
ULONGLONG GetHttpFileSize( LPCTSTR szUrl)
{
 CString strUrl = szUrl;
 strUrl.Replace(_T("//"),_T("/"));

 CInternetSession Session;
 CHttpFile *pHttpFile = NULL;   
 CString strServerName;         
 CString strObject;            // 查询对象名(http文件)
 INTERNET_PORT nPort;          // 端口
 DWORD dwServiceType;          // 服务类型
 DWORD dwHttpRequestFlags = INTERNET_FLAG_NO_AUTO_REDIRECT | INTERNET_FLAG_RELOAD | INTERNET_FLAG_DONT_CACHE;

 const TCHAR szHeaders[]=_T("Accept: */* User-Agent:HttpClient ");

 CHttpConnection* pServer = NULL;

 //网址分析
 BOOL bOK = AfxParseURL(strUrl, dwServiceType, strServerName, strObject, nPort);
 bOK = bOK && (dwServiceType == INTERNET_SERVICE_HTTP);
 if (FALSE == bOK)
 {
  return -1;
 }

 //连接服务器
 pServer = Session.GetHttpConnection(strServerName,nPort);
 pHttpFile = pServer->OpenRequest(CHttpConnection::HTTP_VERB_GET, strObject, NULL, 1, NULL, NULL, dwHttpRequestFlags);
 pHttpFile->AddRequestHeaders(szHeaders);

 pHttpFile->SendRequest();
 DWORD dwRet;
 pHttpFile->QueryInfoStatusCode(dwRet);
 char buff[32];
 ULONGLONG nTotalSize = 0;
 DWORD dwLength = sizeof(nTotalSize);
 if (!pHttpFile->QueryInfo(HTTP_QUERY_CONTENT_LENGTH | HTTP_QUERY_FLAG_NUMBER,&nTotalSize, &dwLength))
 {
  return -1;
 }

 if (pHttpFile != NULL)
 {
  pHttpFile->Close();
 }
 if (pServer != NULL)
 {
  pServer ->Close();
 }
 return nTotalSize;
}