我正在尝试从公共(public) URL 下载一个大文件。起初它似乎工作正常，但十分之一的计算机似乎超时。我最初的尝试是使用 WebClient.DownloadFileAsync，但因为它永远无法完成，所以我转而使用 WebRequest.Create 并直接读取响应流。

我使用 WebRequest.Create 的第一个版本发现了与 WebClient.DownloadFileAsync 相同的问题。操作超时，文件未完成。

如果下载超时，我的下一个版本会重试。这是它变得奇怪的地方。下载最终会以 1 次重试完成最后 7092 个字节。所以下载的文件大小完全相同，但文件已损坏并且与源文件不同。现在我希望损坏发生在最后 7092 个字节中，但事实并非如此。

使用 BeyondCompare 我发现损坏的文件中缺少 2 个字节 block ，总计缺少 7092 个字节!这个丢失的字节位于 1CA49FF0 和 1E31F380，在下载超时并重新启动之前。

这里可能发生了什么？关于如何进一步追踪此问题的任何提示？

这是有问题的代码。

public void DownloadFile(string sourceUri, string destinationPath)
{
    //roughly based on: http://stackoverflow.com/questions/2269607/how-to-programmatically-download-a-large-file-in-c-sharp
    //not using WebClient.DownloadFileAsync as it seems to stall out on large files rarely for unknown reasons.

    using (var fileStream = File.Open(destinationPath, FileMode.Create, FileAccess.Write, FileShare.Read))
    {
        long totalBytesToReceive = 0;
        long totalBytesReceived = 0;
        int attemptCount = 0;
        bool isFinished = false;

        while (!isFinished)
        {
            attemptCount += 1;

            if (attemptCount > 10)
            {
                throw new InvalidOperationException("Too many attempts to download. Aborting.");
            }

            try
            {
                var request = (HttpWebRequest)WebRequest.Create(sourceUri);

                request.Proxy = null;//http://stackoverflow.com/questions/754333/why-is-this-webrequest-code-slow/935728#935728
                _log.AddInformation("Request #{0}.", attemptCount);

                //continue downloading from last attempt.
                if (totalBytesReceived != 0)
                {
                    _log.AddInformation("Request resuming with range: {0} , {1}", totalBytesReceived, totalBytesToReceive);
                    request.AddRange(totalBytesReceived, totalBytesToReceive);
                }

                using (var response = request.GetResponse())
                {
                    _log.AddInformation("Received response. ContentLength={0} , ContentType={1}", response.ContentLength, response.ContentType);

                    if (totalBytesToReceive == 0)
                    {
                        totalBytesToReceive = response.ContentLength;
                    }

                    using (var responseStream = response.GetResponseStream())
                    {
                        _log.AddInformation("Beginning read of response stream.");
                        var buffer = new byte[4096];
                        int bytesRead = responseStream.Read(buffer, 0, buffer.Length);
                        while (bytesRead > 0)
                        {
                            fileStream.Write(buffer, 0, bytesRead);
                            totalBytesReceived += bytesRead;
                            bytesRead = responseStream.Read(buffer, 0, buffer.Length);
                        }

                        _log.AddInformation("Finished read of response stream.");
                    }
                }

                _log.AddInformation("Finished downloading file.");
                isFinished = true;
            }
            catch (Exception ex)
            {
                _log.AddInformation("Response raised exception ({0}). {1}", ex.GetType(), ex.Message);
            }
        }
    }
}

这是损坏下载的日志输出:

Request #1.
Received response. ContentLength=939302925 , ContentType=application/zip
Beginning read of response stream.
Response raised exception (System.Net.WebException). The operation has timed out.
Request #2.
Request resuming with range: 939295833 , 939302925
Received response. ContentLength=7092 , ContentType=application/zip
Beginning read of response stream.
Finished read of response stream.
Finished downloading file.

最佳答案

这是我通常使用的方法，到目前为止，对于您需要的相同类型的加载，它并没有让我失望。尝试使用我的代码稍微更改一下您的代码，看看是否有帮助。

if (!Directory.Exists(localFolder))
{
    Directory.CreateDirectory(localFolder);   
}


try
{
    HttpWebRequest httpRequest = (HttpWebRequest)WebRequest.Create(Path.Combine(uri, filename));
    httpRequest.Method = "GET";

    // if the URI doesn't exist, exception gets thrown here...
    using (HttpWebResponse httpResponse = (HttpWebResponse)httpRequest.GetResponse())
    {
        using (Stream responseStream = httpResponse.GetResponseStream())
        {
            using (FileStream localFileStream = 
                new FileStream(Path.Combine(localFolder, filename), FileMode.Create))
            {
                var buffer = new byte[4096];
                long totalBytesRead = 0;
                int bytesRead;

                while ((bytesRead = responseStream.Read(buffer, 0, buffer.Length)) > 0)
                {
                    totalBytesRead += bytesRead;
                    localFileStream.Write(buffer, 0, bytesRead);
                }
            }
        }
    }
}
catch (Exception ex)
{        
    throw;
}

关于c# - WebRequest 无法正确下载大文件(~ 1 GB)，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/13769027/