目录

• 前言
• 文件下载-服务端
• 使用a标签提供文件下载
• 使用Response.TransmitFile提供文件下载
• 其他方式文件下载
• 文件下载-客户端
• 直接下载
• 异步下载
• 断点续传
• 断点续传(服务端的支持)
• 多线程同时下载（分片下载）

<a href="/新建文件夹2.rar" rel="external nofollow" >下载</a>

//文件下载
public void FileDownload5()
{          
    //前面可以做用户登录验证、用户权限验证等。

    string filename = "大数据.rar";   //客户端保存的文件名  
    string filePath = Server.MapPath("/App_Data/大数据.rar");//要被下载的文件路径 

    Response.ContentType = "application/octet-stream";  //二进制流
    Response.AddHeader("Content-Disposition", "attachment;filename=" + filename);
    Response.TransmitFile(filePath); //将指定文件写入 HTTP 响应输出流
}

 public void FileDownload2()
 {
     string fileName = "新建文件夹2.rar";//客户端保存的文件名  
     string filePath = Server.MapPath("/App_Data/新建文件夹2.rar");//要被下载的文件路径   

     Response.ContentType = "application/octet-stream";//二进制流
     //通知浏览器下载文件而不是打开  
     Response.AddHeader("Content-Disposition", "attachment;  filename=" + HttpUtility.UrlEncode(fileName, System.Text.Encoding.UTF8));

     //以字符流的形式下载文件  
     using (FileStream fs = new FileStream(filePath, FileMode.Open, FileAccess.Read))
     {
         Response.AddHeader("Content-Length", fs.Length.ToString());
         //这里容易内存溢出
         //理论上数组最大长度 int.MaxValue 2147483647 
         //（实际分不到这么多，不同的程序能分到值也不同，本人机器，winfrom( 2147483591 相差56)、iis(也差不多2G)、iis Express（只有100多MB））
         byte[] bytes = new byte[(int)fs.Length];
         fs.Read(bytes, 0, bytes.Length);
         Response.BinaryWrite(bytes);
     }
     Response.Flush();
     Response.End();
 }

public void FileDownload3()
{
    string fileName = "新建文件夹2.rar";//客户端保存的文件名  
    string filePath = Server.MapPath("/App_Data/新建文件夹2.rar");//要被下载的文件路径  
    FileInfo fileInfo = new FileInfo(filePath);
    Response.Clear();
    Response.ClearContent();
    Response.ClearHeaders();
    Response.AddHeader("Content-Disposition", "attachment;filename=\"" + HttpUtility.UrlEncode(fileName, System.Text.Encoding.UTF8) + "\"");
    Response.AddHeader("Content-Length", fileInfo.Length.ToString());//文件大小
    Response.AddHeader("Content-Transfer-Encoding", "binary");
    Response.ContentType = "application/octet-stream";
    Response.WriteFile(fileInfo.FullName);//大小参数必须介于零和最大的 Int32 值之间(也就是最大2G，不过这个操作非常耗内存)
    //这里容易内存溢出
    Response.Flush();
    Response.End();
}

public void FileDownload4()
{
    string fileName = "大数据.rar";//客户端保存的文件名  
    string filePath = Server.MapPath("/App_Data/大数据.rar");//要被下载的文件路径   

    if (System.IO.File.Exists(filePath))
    {
        const long ChunkSize = 102400; //100K 每次读取文件，只读取100K，这样可以缓解服务器的压力  
        byte[] buffer = new byte[ChunkSize];

        Response.Clear();
        using (FileStream fileStream = System.IO.File.OpenRead(filePath))
        {
            long fileSize = fileStream.Length; //文件大小  
            Response.ContentType = "application/octet-stream"; //二进制流
            Response.AddHeader("Content-Disposition", "attachment; filename=" + HttpUtility.UrlEncode(fileName, System.Text.Encoding.UTF8));
            Response.AddHeader("Content-Length", fileStream.Length.ToString());//文件总大小
            while (fileSize > 0 && Response.IsClientConnected)//判断客户端是否还连接了服务器
            {
                //实际读取的大小  
                int readSize = fileStream.Read(buffer, 0, Convert.ToInt32(ChunkSize));
                Response.OutputStream.Write(buffer, 0, readSize);
                Response.Flush();//如果客户端 暂停下载时，这里会阻塞。
                fileSize = fileSize - readSize;//文件剩余大小
            }
        }
        Response.Close();
    }
}

public void FileDownload5()
{          
    //前面可以做用户登录验证、用户权限验证等。

    string filename = "大数据.rar";   //客户端保存的文件名  
    string filePath = Server.MapPath("/App_Data/大数据.rar");//要被下载的文件路径 

    Response.ContentType = "application/octet-stream";  //二进制流
    Response.AddHeader("Content-Disposition", "attachment;filename=" + filename);
    Response.TransmitFile(filePath); //将指定文件写入 HTTP 响应输出流
}

private async void button1_ClickAsync(object sender, EventArgs e)
{
    using (HttpClient http = new HttpClient())
    {
        var httpResponseMessage = await http.GetAsync("http://localhost:813/新建文件夹2.rar");//发送请求 (链接是a标签提供的)
        var contentLength = httpResponseMessage.Content.Headers.ContentLength;//读取文件大小
        using (var stream = await httpResponseMessage.Content.ReadAsStreamAsync())//读取文件流
        {
            var readLength = 1024000;//1000K  每次读取大小
            byte[] bytes = new byte[readLength];
            int writeLength;
            using (FileStream fs = new FileStream(Application.StartupPath + "/temp.rar", FileMode.Append, FileAccess.Write))//使用追加方式打开一个文件流
            {
                while ((writeLength = stream.Read(bytes, 0, readLength)) > 0)//分块读取文件流
                {
                    fs.Write(bytes, 0, writeLength);//追加写入文件
                    contentLength -= writeLength;
                    if (contentLength == 0)//如果写入完成 给出提示
                        MessageBox.Show("下载完成");
                }
            }
        }
    } 
}

var httpResponseMessage = await http.GetAsync("http://localhost:813/新建文件夹2.rar");//发送请求

var httpResponseMessage = await http.GetAsync("http://localhost:813/新建文件夹2.rar",HttpCompletionOption.ResponseHeadersRead);//响应一可用且标题可读时即应完成的操作。 (尚未读取的内容。)

/// <summary>
/// 异步下载
/// </summary>
/// <param name="sender"></param>
/// <param name="e"></param>
private async void button2_ClickAsync(object sender, EventArgs e)
{
    //开启一个异步线程
    await Task.Run(async () =>
    {
        //异步操作UI元素
        label1.Invoke((Action)(() =>
                {
                    label1.Text = "准备下载...";
                }));

        long downloadSize = 0;//已经下载大小
        long downloadSpeed = 0;//下载速度
        using (HttpClient http = new HttpClient())
        {
            var httpResponseMessage = await http.GetAsync("http://localhost:813/新建文件夹2.rar", HttpCompletionOption.ResponseHeadersRead);//发送请求
            var contentLength = httpResponseMessage.Content.Headers.ContentLength;   //文件大小                
            using (var stream = await httpResponseMessage.Content.ReadAsStreamAsync())
            {
                var readLength = 1024000;//1000K
                byte[] bytes = new byte[readLength];
                int writeLength;
                var beginSecond = DateTime.Now.Second;//当前时间秒
                //使用追加方式打开一个文件流
                using (FileStream fs = new FileStream(Application.StartupPath + "/temp.rar", FileMode.Append, FileAccess.Write))
                {
                    while ((writeLength = stream.Read(bytes, 0, readLength)) > 0)
                    {                 
                         fs.Write(bytes, 0, writeLength);                
                        downloadSize += writeLength;
                        downloadSpeed += writeLength;
                        progressBar1.Invoke((Action)(() =>
                        {
                            var endSecond = DateTime.Now.Second;
                            if (beginSecond != endSecond)//计算速度
                            {
                                downloadSpeed = downloadSpeed / (endSecond - beginSecond);
                                label1.Text = "下载速度" + downloadSpeed / 1024 + "KB/S";

                                beginSecond = DateTime.Now.Second;
                                downloadSpeed = 0;//清空
                            }
                            progressBar1.Value = Math.Max((int)(downloadSize * 100 / contentLength), 1);
                        }));
                    }
               }
                label1.Invoke((Action)(() =>
                {
                    label1.Text = "下载完成";
                }));
            }
        }
    });
}

var request = new HttpRequestMessage { RequestUri = new Uri(url) };
request.Headers.Range = new RangeHeaderValue(rangeBegin, null); //【关键点】全局变量记录已经下载了多少，然后下次从这个位置开始下载。
var httpResponseMessage = await http.SendAsync(request, HttpCompletionOption.ResponseHeadersRead);

/// <summary>
/// 是否暂停
/// </summary>
static bool isPause = true;
/// <summary>
/// 下载开始位置（也就是已经下载了的位置）
/// </summary>
static long rangeBegin = 0; //(当然，这个值也可以存为持久化。如文本、数据库等)

private async void button3_ClickAsync(object sender, EventArgs e)
{
    isPause = !isPause;
    if (!isPause)//点击下载
    {
        button3.Text = "暂停";

        await Task.Run(async () =>
        {
            //异步操作UI元素
            label1.Invoke((Action)(() =>
           {
               label1.Text = "准备下载...";
           }));

            long downloadSpeed = 0;//下载速度
            using (HttpClient http = new HttpClient())
            {
                var url = "http://localhost:813/新建文件夹2.rar";
                var request = new HttpRequestMessage { RequestUri = new Uri(url) };
                request.Headers.Range = new RangeHeaderValue(rangeBegin, null); //【关键点】全局变量记录已经下载了多少，然后下次从这个位置开始下载。
                var httpResponseMessage = await http.SendAsync(request, HttpCompletionOption.ResponseHeadersRead);
                var contentLength = httpResponseMessage.Content.Headers.ContentLength;//本次请求的内容大小
                if (httpResponseMessage.Content.Headers.ContentRange != null) //如果为空，则说明服务器不支持断点续传
                {
                    contentLength = httpResponseMessage.Content.Headers.ContentRange.Length;//服务器上的文件大小
                }

                using (var stream = await httpResponseMessage.Content.ReadAsStreamAsync())
                {
                    var readLength = 1024000;//1000K
                    byte[] bytes = new byte[readLength];
                    int writeLength;
                    var beginSecond = DateTime.Now.Second;//当前时间秒
                    while ((writeLength = stream.Read(bytes, 0, readLength)) > 0 && !isPause)
                    {
                        //使用追加方式打开一个文件流
                        using (FileStream fs = new FileStream(Application.StartupPath + "/temp.rar", FileMode.Append, FileAccess.Write))
                        {
                            fs.Write(bytes, 0, writeLength);
                        }
                        downloadSpeed += writeLength;
                        rangeBegin += writeLength;
                        progressBar1.Invoke((Action)(() =>
                        {
                            var endSecond = DateTime.Now.Second;
                            if (beginSecond != endSecond)//计算速度
                            {
                                downloadSpeed = downloadSpeed / (endSecond - beginSecond);
                                label1.Text = "下载速度" + downloadSpeed / 1024 + "KB/S";

                                beginSecond = DateTime.Now.Second;
                                downloadSpeed = 0;//清空
                            }
                            progressBar1.Value = Math.Max((int)((rangeBegin) * 100 / contentLength), 1);
                        }));
                    }

                    if (rangeBegin == contentLength)
                    {
                        label1.Invoke((Action)(() =>
                        {
                            label1.Text = "下载完成";
                        }));
                    }
                }
            }
        });
    }
    else//点击暂停
    {
        button3.Text = "继续下载";
        label1.Text = "暂停下载";
    }
}

public void FileDownload5()
{          
    //前面可以做用户登录验证、用户权限验证等。

    string filename = "大数据.rar";   //客户端保存的文件名  
    string filePath = Server.MapPath("/App_Data/大数据.rar");//要被下载的文件路径 

    var range = Request.Headers["Range"];
    if (!string.IsNullOrWhiteSpace(range))//如果遵守协议，支持断点续传
    {
        var fileLength = new FileInfo(filePath).Length;//文件的总大小
        long begin;//文件的开始位置
        long end;//文件的结束位置
        long.TryParse(range.Split(''='')[1].Split(''-'')[0], out begin);
        long.TryParse(range.Split(''-'')[1], out end);
        end = end - begin > 0 ? end : (fileLength - 1);// 如果没有结束位置，那我们读剩下的全部

        //表头 表明  下载文件的开始、结束位置 和文件总大小
        Response.AddHeader("Content-Range", "bytes " + begin + "-" + end + "/" + fileLength);
        Response.ContentType = "application/octet-stream";
        Response.AddHeader("Content-Disposition", "attachment;filename=" + filename);
        Response.TransmitFile(filePath, begin, (end - begin));//发送 文件开始位置读取的大小
    }
    else
    {
        Response.ContentType = "application/octet-stream";
        Response.AddHeader("Content-Disposition", "attachment;filename=" + filename);
        Response.TransmitFile(filePath);
    }
}

• c# 断点续传的实现
• C# FileStream实现多线程断点续传
• C# 文件下载之断点续传实现代码
• C#文件断点续传实现方法
• C#实现文件断点续传下载的方法
• c#实现断点续传功能示例分享
• C#实现支持断点续传多线程下载客户端工具类
• C#服务端图片打包下载实现代码解析
• c# 实现文件上传下载功能的实例代码

目录

• 协议梳理
• 实现步骤
• 写在最后

import ''package:dio/dio.dart'';

typedef ProgressCallBack = void Function(int count, int total);

typedef CancelTokenProvider = void Function(CancelToken cancelToken);

abstract class AssetRepositoryProtocol {
  /// 下载单一资源
  Future<String> downloadAsset(String url,
      {String? subDir,
      ProgressCallBack? onReceiveProgress,
      CancelTokenProvider? cancelTokenProvider,
      Function(String)? done,
      Function(Exception)? failed});

  /// 取消下载，Dio中通过CancelToken可控制
  void cancelDownload(CancelToken cancelToken);

  /// 获取文件的缓存地址
  Future<String?> filePathForAsset(String url, {String? subDir});

  /// 检查文件是否缓存成功，简单对比md5
  Future<String?> checkCachedSuccess(String url, {String? md5Str});
  
  /// 查看缓存文件的大小
  Future<int> cachedFileSize({String? subDir});

  /// 清除缓存
  Future<void> clearCache({String? subDir});
}

class AssetRepository implements AssetRepositoryProtocol {
  AssetRepository(this.httpManager);

  final HttpManagerProtocol httpManager;

  @override
  Future<String> downloadAsset(String url,
      {String? subDir,
      ProgressCallBack? onReceiveProgress,
      CancelTokenProvider? cancelTokenProvider,
      Function(String)? done,
      Function(Exception)? failed}) async {
    CancelToken cancelToken = CancelToken();
    if (cancelTokenProvider != null) {
      cancelTokenProvider(cancelToken);
    }

    final savePath = await _getSavePath(url, subDir: subDir);
    try {
      httpManager.downloadFile(
          url: url,
          savePath: savePath + ''.temp'',
          onReceiveProgress: onReceiveProgress,
          cancelToken: cancelToken,
          done: () {
            done?.call(savePath);
          },
          failed: (e) {
            print(e);
            failed?.call(e);
          });
      return savePath;
    } catch (e) {
      print(e);
      rethrow;
    }
  }

  @override
  void cancelDownload(CancelToken cancelToken) {
    try {
      if (!cancelToken.isCancelled) {
        cancelToken.cancel();
      }
    } catch (e) {
      print(e);
    }
  }

  @override
  Future<String?> filePathForAsset(String url, {String? subDir}) async {
    final path = await _getSavePath(url, subDir: subDir);
    final file = File(path);
    if (!(await file.exists())) {
      return null;
    }
    return path;
  }

  @override
  Future<String?> checkCachedSuccess(String url, {String? md5Str}) async {
    String? path = await _getSavePath(url, subDir: FileType.video.dirName);
    bool isCached = await File(path).exists();
    if (isCached && (md5Str != null && md5Str.isNotEmpty)) {
      // 存在但是md5验证不通过
      File(path).readAsBytes().then((Uint8List str) {
        if (md5.convert(str).toString() != md5Str) {
          path = null;
        }
      });
    } else if (isCached) {
      return path;
    } else {
      path = null;
    }
    return path;
  }
  
  @override
  Future<int> cachedFileSize({String? subDir}) async {
    final dir = await _getDir(subDir: subDir);
    if (!(await dir.exists())) {
      return 0;
    }

    int totalSize = 0;
    await for (var entity in dir.list(recursive: true)) {
      if (entity is File) {
        try {
          totalSize += await entity.length();
        } catch (e) {
          print(''Get size of $entity failed with exception: $e'');
        }
      }
    }

    return totalSize;
  }

  @override
  Future<void> clearCache({String? subDir}) async {
    final dir = await _getDir(subDir: subDir);
    if (!(await dir.exists())) {
      return;
    }
    dir.deleteSync(recursive: true);
  }

  Future<String> _getSavePath(String url, {String? subDir}) async {
    final saveDir = await _getDir(subDir: subDir);

    if (!saveDir.existsSync()) {
      saveDir.createSync(recursive: true);
    }

    final uri = Uri.parse(url);
    final fileName = uri.pathSegments.last;
    return saveDir.path + fileName;
  }

  Future<Directory> _getDir({String? subDir}) async {
    final cacheDir = await getTemporaryDirectory();
    late final Directory saveDir;
    if (subDir == null) {
      saveDir = cacheDir;
    } else {
      saveDir = Directory(cacheDir.path + ''/$subDir/'');
    }
    return saveDir;
  }
}

final downloadDio = Dio();

Future<void> downloadFile({
  required String url,
  required String savePath,
  required CancelToken cancelToken,
  ProgressCallback? onReceiveProgress,
  void Function()? done,
  void Function(Exception)? failed,
}) async {
  int downloadStart = 0;
  File f = File(savePath);
  if (await f.exists()) {
    // 文件存在时拿到已下载的字节数
    downloadStart = f.lengthSync();
  }
  print("start: $downloadStart");
  try {
    var response = await downloadDio.get<ResponseBody>(
      url,
      options: Options(
        /// Receive response data as a stream
        responseType: ResponseType.stream,
        followRedirects: false,
        headers: {
          /// 加入range请求头，实现断点续传
          "range": "bytes=$downloadStart-",
        },
      ),
    );
    File file = File(savePath);
    RandomAccessFile raf = file.openSync(mode: FileMode.append);
    int received = downloadStart;
    int total = await _getContentLength(response);
    Stream<Uint8List> stream = response.data!.stream;
    StreamSubscription<Uint8List>? subscription;
    subscription = stream.listen(
      (data) {
        /// Write files must be synchronized
        raf.writeFromSync(data);
        received += data.length;
        onReceiveProgress?.call(received, total);
      },
      onDone: () async {
        file.rename(savePath.replaceAll(''.temp'', ''''));
        await raf.close();
        done?.call();
      },
      onError: (e) async {
        await raf.close();
        failed?.call(e);
      },
      cancelOnError: true,
    );
    cancelToken.whenCancel.then((_) async {
      await subscription?.cancel();
      await raf.close();
    });
  } on DioError catch (error) {
    if (CancelToken.isCancel(error)) {
      print("Download cancelled");
    } else {
      failed?.call(error);
    }
  }
}

• Flutter Http分块下载与断点续传的实现
• Android实现断点续传功能
• Android快速实现断点续传的方法
• android实现多线程下载文件（支持暂停、取消、断点续传）
• Android 断点续传原理以及实现

/*
 * PHP下载断点续传
 */
function dl_file_resume($file){ 
  
    //检测文件是否存在 
    if (!is_file($file)) { die("<b>404 File not found!</b>"); } 
       
       
    $len = filesize($file);//获取文件大小 
    $filename = basename($file);//获取文件名字 
    $file_extension = strtolower(substr(strrchr($filename,"."),1));//获取文件扩展名 
       
    //根据扩展名 指出输出浏览器格式 
    switch( $file_extension ) { 
        case "exe": $ctype="application/octet-stream"; break; 
        case "zip": $ctype="application/zip"; break; 
        case "mp3": $ctype="audio/mpeg"; break; 
        case "mpg":$ctype="video/mpeg"; break; 
        case "avi": $ctype="video/x-msvideo"; break; 
        default: $ctype="application/force-download"; 
    } 
       
    //Begin writing headers 
    header("Cache-Control:"); 
    header("Cache-Control: public"); 
       
    //设置输出浏览器格式 
    header("Content-Type: $ctype"); 
    if (strstr($_SERVER[&#39;HTTP_USER_AGENT&#39;], "MSIE")) {//如果是IE浏览器 
        # workaround for IE filename bug with multiple periods / multiple dots in filename 
        # that adds square brackets to filename - eg. setup.abc.exe becomes setup[1].abc.exe 
        $iefilename = preg_replace(&#39;/\./&#39;, &#39;%2e&#39;, $filename, substr_count($filename, &#39;.&#39;) - 1); 
        header("Content-Disposition: attachment; filename=\"$iefilename\""); 
    } else { 
        header("Content-Disposition: attachment; filename=\"$filename\""); 
    } 
    header("Accept-Ranges: bytes"); 
       
    $size=filesize($file); 
    //如果有$_SERVER[&#39;HTTP_RANGE&#39;]参数 
    if(isset($_SERVER[&#39;HTTP_RANGE&#39;])) { 
/*   --------------------------- 
   Range头域 　　Range头域可以请求实体的一个或者多个子范围。例如， 　　表示头500个字节：bytes=0-499 　

　表示第二个500字节：bytes=500-999 　　表示最后500个字节：bytes=-500 　　表示500字节以后的范围：

bytes=500- 　　第一个和最后一个字节：bytes=0-0,-1 　　同时指定几个范围：bytes=500-600,601-999 　　但是

服务器可以忽略此请求头，如果无条件GET包含Range请求头，响应会以状态码206（PartialContent）返回而不是以

200 （OK）。 
   ---------------------------*/
  
     
// 断点后再次连接 $_SERVER[&#39;HTTP_RANGE&#39;] 的值 bytes=4390912- 
     
        list($a, $range)=explode("=",$_SERVER[&#39;HTTP_RANGE&#39;]); 
   //if yes, download missing part 
        str_replace($range, "-", $range);//这句干什么的呢。。。。 
   $size2=$size-1;//文件总字节数 
        $new_length=$size2-$range;//获取下次下载的长度 
        header("HTTP/1.1 206 Partial Content"); 
        header("Content-Length: $new_length");//输入总长 
        header("Content-Range: bytes $range$size2/$size");
		//Content-Range: bytes 4908618-4988927/4988928   95%的时候 
    } else {//第一次连接 
        $size2=$size-1; 
        header("Content-Range: bytes 0-$size2/$size"); //Content-Range: bytes 0-4988927/4988928 
        header("Content-Length: ".$size);//输出总长 
    } 
    //打开文件 
    $fp=fopen("$file","rb"); 
    //设置指针位置 
    fseek($fp,$range); 
    //虚幻输出 
    while(!feof($fp)){ 
        //设置文件最长执行时间 
        set_time_limit(0); 
        print(fread($fp,1024*8));//输出文件 
        flush();//输出缓冲 
        ob_flush(); 
    } 
    fclose($fp); 
    exit; 
} 
  
  
dl_file_resume("aa.zip");//同级目录的1.zip 文件