代码片段

 

/// <summary>
    /// 下载文件，支持大文件、续传、速度限制。支持续传的响应头Accept-Ranges、ETag，请求头Range 。
    /// Accept-Ranges：响应头，向客户端指明，此进程支持可恢复下载.实现后台智能传输服务（BITS），值为：bytes；
    /// ETag：响应头，用于对客户端的初始（200）响应，以及来自客户端的恢复请求，
    /// 必须为每个文件提供一个唯一的ETag值（可由文件名和文件最后被修改的日期组成），这使客户端软件能够验证它们已经下载的字节块是否仍然是最新的。
    /// Range：续传的起始位置，即已经下载到客户端的字节数，值如：bytes=1474560- 。
    /// 另外：UrlEncode编码后会把文件名中的空格转换中+（+转换为%2b），但是浏览器是不能理解加号为空格的，所以在浏览器下载得到的文件，空格就变成了加号；
    /// 解决办法：UrlEncode 之后, 将 "+" 替换成 "%20"，因为浏览器将%20转换为空格
    /// </summary>
    /// <param name="httpContext">当前请求的HttpContext</param>
    /// <param name="filePath">下载文件的物理路径，含路径、文件名</param>
    /// <param name="speed">下载速度：每秒允许下载的字节数</param>
    /// <returns>true下载成功，false下载失败</returns>
    public static bool DownloadFile(HttpContext httpContext, string filePath, long speed)
    {
        httpContext.Response.Clear();
        bool ret = true;
        try
        {
            #region --验证：HttpMethod，请求的文件是否存在#region
            switch (httpContext.Request.HttpMethod.ToUpper())
            { //目前只支持GET和HEAD方法
                case "GET":
                case "HEAD":
                    break;
                default:
                    httpContext.Response.StatusCode = 501;
                    return false;
            }
            if (!File.Exists(filePath))
            {
                httpContext.Response.StatusCode = 404;
                return false;
            }
            #endregion

            #region 定义局部变量#region 定义局部变量
            long startBytes = 0;
            long stopBytes = 0;
            int packSize = 1024 * 10; //分块读取，每块10K bytes
            string fileName = Path.GetFileName(filePath);
            FileStream myFile = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
            BinaryReader br = new BinaryReader(myFile);
            long fileLength = myFile.Length;

            int sleep = (int)Math.Ceiling(1000.0 * packSize / speed);//毫秒数：读取下一数据块的时间间隔
            string lastUpdateTiemStr = File.GetLastWriteTimeUtc(filePath).ToString("r");
            string eTag = HttpUtility.UrlEncode(fileName, Encoding.UTF8) + lastUpdateTiemStr;//便于恢复下载时提取请求头;
            #endregion

            #region --验证：文件是否太大，是否是续传，且在上次被请求的日期之后是否被修改过
            if (myFile.Length > long.MaxValue)
            {//-------文件太大了-------
                httpContext.Response.StatusCode = 413;//请求实体太大
                return false;
            }

            if (httpContext.Request.Headers["If-Range"] != null)//对应响应头ETag：文件名+文件最后修改时间
            {
                //----------上次被请求的日期之后被修改过--------------
                if (httpContext.Request.Headers["If-Range"].Replace("\"", "") != eTag)
                {//文件修改过
                    httpContext.Response.StatusCode = 412;//预处理失败
                    return false;
                }
            }
            #endregion

            try
            {
                #region -------添加重要响应头、解析请求头、相关验证
                httpContext.Response.Clear();

                if (httpContext.Request.Headers["Range"] != null)
                {//------如果是续传请求，则获取续传的起始位置，即已经下载到客户端的字节数------
                    httpContext.Response.StatusCode = 206;//重要：续传必须，表示局部范围响应。初始下载时默认为200
                    string[] range = httpContext.Request.Headers["Range"].Split(new char[] { '=', '-' });//"bytes=1474560-"
                    startBytes = Convert.ToInt64(range[1]);//已经下载的字节数，即本次下载的开始位置 
                    if (startBytes < 0 || startBytes >= fileLength)
                    {//无效的起始位置
                        return false;
                    }
                    if (range.Length == 3)
                    {
                        stopBytes = Convert.ToInt64(range[2]);//结束下载的字节数，即本次下载的结束位置 
                        if (startBytes < 0 || startBytes >= fileLength)
                        {
                            return false;
                        }
                    }
                }

                httpContext.Response.Buffer = false;
                //httpContext.Response.AddHeader("Content-MD5", My_Des.MD5JiaMi_File(filePath));//用于验证文件
                httpContext.Response.AddHeader("Accept-Ranges", "bytes");//重要：续传必须
                httpContext.Response.AppendHeader("ETag", "\"" + eTag + "\"");//重要：续传必须
                httpContext.Response.AppendHeader("Last-Modified", lastUpdateTiemStr);//把最后修改日期写入响应               
                httpContext.Response.ContentType = "application/octet-stream";//MIME类型：匹配任意文件类型
                httpContext.Response.AddHeader("Content-Disposition", "attachment;filename=" + HttpUtility.UrlEncode(fileName, Encoding.UTF8).Replace("+", "%20"));
                httpContext.Response.AddHeader("Content-Length", (fileLength - startBytes).ToString());
                httpContext.Response.AddHeader("Connection", "Keep-Alive");
                httpContext.Response.ContentEncoding = Encoding.UTF8;
                if (startBytes > 0)
                {//------如果是续传请求，告诉客户端本次的开始字节数，总长度，以便客户端将续传数据追加到startBytes位置后----------
                    httpContext.Response.AddHeader("Content-Range", string.Format("bytes {0}-{1}/{2}", startBytes, fileLength - 1, fileLength));
                }
                #endregion

                #region -------向客户端发送数据块-------------------
                br.BaseStream.Seek(startBytes, SeekOrigin.Begin);
                int maxCount = (int)Math.Ceiling((fileLength - startBytes + 0.0) / packSize);//分块下载，剩余部分可分成的块数
                for (int i = 0; i < maxCount && httpContext.Response.IsClientConnected; i++)
                {//客户端中断连接，则暂停
                    httpContext.Response.BinaryWrite(br.ReadBytes(packSize));
                    httpContext.Response.Flush();
                    if (sleep > 1) Thread.Sleep(sleep);
                }
                #endregion
            }
            catch
            {
                ret = false;
            }
            finally
            {
                br.Close();
                myFile.Close();
            }
        }
        catch
        {
            ret = false;
        }
        return ret;
    }