断开网络连接后，为什么可以使用CHttpConnection和CHttpFile下载文件 [英] Why can I download files with CHttpConnection and CHttpFile when disconnect the network

问题描述

BOOL Download(CString& m_strDownLoadURL,CString& m_strDownLoadDesFileName)
{
        ASSERT(m_strDownLoadURL != "");
	ASSERT(m_strDownLoadDesFileName != "");
        CInternetSession session;
	CHttpConnection* pHttpConnection = NULL;
	CHttpFile* pHttpFile = NULL;
	CString strServer, strObject;
	INTERNET_PORT wPort;
	DWORD dwType;
	const int nTimeOut = 2000;
	session.SetOption(INTERNET_OPTION_CONNECT_TIMEOUT, nTimeOut); 	session.SetOption(INTERNET_OPTION_CONNECT_RETRIES, 1);  
	char* pszBuffer = NULL; 
        	try
	       {
		AfxParseURL(m_strDownLoadURL, dwType, strServer, strObject, wPort);
		pHttpConnection = session.GetHttpConnection(strServer, wPort);
		pHttpFile = pHttpConnection->OpenRequest(CHttpConnection::HTTP_VERB_GET, strObject);
		if(pHttpFile->SendRequest() == FALSE)
			return false;
		DWORD dwStateCode;
               pHttpFile->QueryInfoStatusCode(dwStateCode);
		if(dwStateCode == HTTP_STATUS_OK)
		{
		  HANDLE hFile = CreateFile(m_strDownLoadDesFileName, GENERIC_WRITE,
				FILE_SHARE_WRITE, NULL,                              CREATE_ALWAYS,                  FILE_ATTRIBUTE_NORMAL,
				NULL);  
			if(hFile == INVALID_HANDLE_VALUE)
			{
				pHttpFile->Close();
				pHttpConnection->Close();
				session.Close();
				return false;
			}

                }
              
			char szInfoBuffer[1000]; 
			DWORD dwFileSize = 0;   
			DWORD dwInfoBufferSize = sizeof(szInfoBuffer);
			BOOL bResult = FALSE;
			bResult = pHttpFile->QueryInfo(HTTP_QUERY_CONTENT_LENGTH,
			(void*)szInfoBuffer,&dwInfoBufferSize,NULL);


                        dwFileSize = atoi(szInfoBuffer);
			const int BUFFER_LENGTH = 1024 * 10;
			pszBuffer = new char[BUFFER_LENGTH];  			                  DWORD dwWrite, dwTotalWrite;
			dwWrite = dwTotalWrite = 0;
			UINT nRead = pHttpFile->Read(pszBuffer, BUFFER_LENGTH);
                        
                        while(nRead > 0)
			{
			    WriteFile(hFile, pszBuffer, nRead, &dwWrite, NULL);  
			    dwTotalWrite += dwWrite;
			    nRead = pHttpFile->Read(pszBuffer, BUFFER_LENGTH);
			}

                        delete[]pszBuffer;
			pszBuffer = NULL;
			CloseHandle(hFile);
              catch(...)
             {
                // do something
             }

推荐答案

如果您已经通过活动的Internet连接检索了一次数据，则会对其进行缓存.如果尝试再次检索相同的数据，则可能会从缓存中获取一个副本.

要强制进行新的下载，请在调用CHttpConnection::OpenRequest()时使用标志INTERNET_FLAG_RELOAD.

If you have retrieved the data once with active internet connection it is cached. If you try to retrieve the same data again, you might get a copy from the cache.

To force a new download, use the flag INTERNET_FLAG_RELOAD when calling CHttpConnection::OpenRequest().

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