C#文件断点续传实现方法攻略

背景和原理介绍

在文件传输中，当传输过程中出现异常，或连接中断，一般需要重新传输。但如果文件太大，重新传输的成本太高，而此时恰好传输过程中已经传输了一部分，通过断点续传可以只续传未完成的部分，可以大幅减少传输成本。文件断点续传实现的原理，在下载时是通过请求服务端时在header部分加上range请求头，标明下载文件的起止断点，服务端则返回此范围内的文件内容。而上传时，可以通过multipart/form-data的方式上传文件分块，并在服务端将分块拼接起来即可实现。

文件下载断点续传实现

方式一：使用HttpWebRequest发送range请求头

在下载文件时，使用HttpWebRequest发送range请求头即可实现文件下载断点续传。具体实现步骤如下：
1. 使用HttpWebRequest构建请求对象
csharp HttpWebRequest request = (HttpWebRequest) WebRequest.Create(url);
2. 使用AddHeader方法添加range请求头，可根据实际需要设置请求范围，如果不设置，则表示从0字节开始下载
csharp request.AddRange(startByte, endByte);
3. 发送请求并返回响应结果
csharp HttpWebResponse response = (HttpWebResponse) request.GetResponse();
4. 读取响应数据流，并将数据写入本地文件中
csharp using (Stream responseStream = response.GetResponseStream()) { using (FileStream fs = new FileStream(filePath, FileMode.Append)) { byte[] buffer = new byte[1024]; int length = 0; while ((length = responseStream.Read(buffer, 0, buffer.Length)) > 0) { fs.Write(buffer, 0, length); } } }
实际代码示例可参见如下代码：
```csharp
public static void DownloadFile(string url, string filePath)
{
HttpWebRequest request = (HttpWebRequest) WebRequest.Create(url);
long fileSize = GetFileSize(url);
request.Timeout = 10000;
request.ReadWriteTimeout = 10000;
request.AddRange(0, fileSize - 1);
HttpWebResponse response = (HttpWebResponse) request.GetResponse();
Stream responseStream = response.GetResponseStream();
byte[] buffer = new byte[1024];
FileStream fs = new FileStream(filePath, FileMode.Create);
int length = 0;
while ((length = responseStream.Read(buffer, 0, buffer.Length)) > 0)
{
fs.Write(buffer, 0, length);
}
fs.Close();
responseStream.Close();
}

public static long GetFileSize(string url)
{
HttpWebRequest request = (HttpWebRequest) WebRequest.Create(url);
request.Method = "HEAD";
HttpWebResponse response = (HttpWebResponse) request.GetResponse();
return response.ContentLength;
}
```

方式二：使用Nuget包实现断点续传下载

使用Nuget包“BrokenEvent.Shared”可以快速实现断点续传下载。示例代码如下：
csharp var downloader = new HttpDownloader(new Uri(url), filePath); downloader.BytesPerSecondSpeedLimit = 50000; downloader.ProgressChanged += (object sender, HttpDownloadProgressChangedEventArgs e) => { Console.Write($"\r{e.ProgressPercentage}% ({e.BytesDownloaded}/{e.TotalBytes})"); }; downloader.Start();

文件上传断点续传实现

在文件上传断点续传实现中，需要将文件分块，分别上传到服务端，服务端将文件分块拼接起来形成完整文件。具体实现步骤如下：
1. 根据文件大小和分块大小计算出需要分多少块

    long fileSize = new FileInfo(filePath).Length;
    int blockCount = (int) Math.Ceiling((double) fileSize / blockSize);

根据分块数量生成临时文件名，将各个分块分别保存在本地

    string tempPath = Path.GetDirectoryName(filePath);
    for (int i = 0; i < blockCount; i++)
    {
        string blockFileName = Path.Combine(tempPath, $"{fileName}.part{i}");
        long length = blockSize;
        if (i == blockCount - 1)
        {
            length = fileSize - i * blockSize;
        }
        byte[] buffer = new byte[length];
        using (FileStream fs = new FileStream(filePath, FileMode.Open, FileAccess.ReadWrite, FileShare.ReadWrite))
        {
            fs.Seek(i * blockSize, SeekOrigin.Begin);
            fs.Read(buffer, 0, (int)length);
        }
        using (FileStream fs = new FileStream(blockFileName, FileMode.Create, FileAccess.Write, FileShare.Write))
        {
            fs.Write(buffer, 0, buffer.Length);
        }
    }

分块上传，每个分块上传时需要带上当前分块的起始位置和长度，服务端需要根据这两个参数将分块拼接起来。如果上传过程中发生异常，可以记录当前上传到哪个分块，下次上传时直接从记录的位置继续上传即可。

    try
    {
        for (int i = 0; i < blockCount; i++)
        {
            string blockFileName = Path.Combine(tempPath, $"{fileName}.part{i}");
            FileInfo file = new FileInfo(blockFileName);
            long startPosition = i * blockSize;
            int length = (int)file.Length;
            byte[] buffer = new byte[length];
            using (FileStream fs = new FileStream(blockFileName, FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
            {
                fs.Read(buffer, 0, buffer.Length);
            }

            HttpWebRequest request = (HttpWebRequest)WebRequest.Create($"{url}?fileName={fileName}&startIndex={startPosition}");
            request.Timeout = timeout;
            request.Method = "POST";

            Stream requestStream = request.GetRequestStream();
            requestStream.Write(buffer, 0, buffer.Length);
            requestStream.Close();

            HttpWebResponse response = (HttpWebResponse)request.GetResponse();
            response.Close();

            finishedBlockNumber = i;
        }
    }
    catch (Exception ex)
    {
        Console.WriteLine($"上传出错：{ex.Message}");
    }

服务端接收分块时需要根据上传的起始位置挨个拼接各个分块，最终形成完整文件

    public void ReceiveFile(string fileName, long startIndex ,Stream fileStream)
    {
        string tempFilePath = Path.Combine(HttpContext.Current.Server.MapPath("~/Upload/Temp"), $"{fileName}.part{startIndex / blockSize}");
        if (!File.Exists(tempFilePath))
        {
            return;
        }
        FileStream fs = new FileStream(tempFilePath, FileMode.Append, FileAccess.Write, FileShare.Write);
        fileStream.CopyTo(fs);
        fs.Close();
        UploadStatus status = new UploadStatus()
        {
            FileName = fileName,
            StartIndex = startIndex,
            EndIndex = startIndex + fileStream.Length
        };
        UpdateUploadStatus(status);
        if (startIndex % blockSize == 0 && status.EndIndex == fileSize)
        {
            CombineBlocks(fileName);
        }
    }

    private void CombineBlocks(string fileName)
    {
        List<string> blockFiles = Directory.GetFiles(Path.Combine(HttpContext.Current.Server.MapPath("~/Upload/Temp"))).ToList();
        blockFiles = blockFiles.Where(f => Path.GetFileName(f).StartsWith($"{fileName}.part")).ToList();
        blockFiles.Sort(new FileNameComparer());
        using (FileStream fs = new FileStream(Path.Combine(savePath, fileName), FileMode.Append, FileAccess.Write, FileShare.Write))
        {
            foreach (string blockFile in blockFiles)
            {
                byte[] buffer = File.ReadAllBytes(blockFile);
                fs.Write(buffer, 0, buffer.Length);
                File.Delete(blockFile);
            }
            fs.Close();
        }
    }

总结

以上是C#文件断点续传实现的攻略，其中包括了文件下载和文件上传两个方面的实现，希望对需要实现文件断点续传功能的开发者有所帮助。

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C#文件断点续传实现方法