原文:

我服务器上文件只能下载一次，下载了之后就不能下载了，大文件或网速不好时，可能服务端文件流发送完了，客户端还没下载完，导致下载失败，

那么最好的办法就是：在续传时不判断（If-Range，或If-Match 不为空时不判断，仍然发送对应的文件流）就行了，这样有一个漏洞，就是一个文件没下载完时，可以同时下载很多次，但是没办法，客户使用浏览器下载不好控制（如果记录用户开始下载的次数，逻辑上也不行—他可以下载到一半，然后时间不够不下载了，下次再重新下载，这在业务上也是允许他没下载完时重新下载的）。

#region 下载文件处理-downfiledeal        /// 
        /// 下载文件处理        /// 
        /// 资源id        /// 用户id        /// 下载类型：1-购买，2-今日免费，3-订阅用户        /// 提示页面titie        /// 资源类型（暂时不考虑）        /// 
               private string downfiledeal(string ResourceID, string UserID, string downtype, string webtitle, string ResourceType = "")        {            TopicInformationBLL bll_tf = new TopicInformationBLL();            TopicInformation tf = bll_tf.GetById(ResourceID);            bll_tf.Dispose();            string FilePath = "";            FilePath = tf.TopicSourceFile.ToLower();            if (FilePath.StartsWith("http://www.***.com"))//不是外网文件夹服务器路径            {                FilePath = FilePath.Replace("http://www.***.com", "");            }            if (!FilePath.StartsWith("/")) FilePath = "/" + FilePath;            FilePath = Server.MapPath(FilePath);            FileInfo DownloadFile = new FileInfo(FilePath);            System.IO.Stream iStream = null;            int downsize = 1024;//读取web.config中的配置            try { downsize = Convert.ToInt32(System.Configuration.ConfigurationManager.AppSettings["downsize"]); }            catch { downsize = 1024; }            int buffersize = 1024 * downsize;//默认每次发送1K;            byte[] buffer = new Byte[buffersize];// Buffer to read 10K bytes in chunk:            int currentlength;// Length of each read to buffer:            long dataToRead;// Total bytes to read:            long dataSended;//已发送的字节            long datalength;//文件字节总长度            string filename = System.IO.Path.GetFileName(FilePath); // Identify the file name.            try            {                System.Collections.Specialized.NameValueCollection hds = Request.Headers;                long startBytes = 0;                Response.Clear();                Response.ClearHeaders();                if (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------                      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 = 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 >= DownloadFile.Length)                    {//无效的起始位置                          startBytes = 0;                    }                }                iStream = new System.IO.FileStream(FilePath, System.IO.FileMode.Open, System.IO.FileAccess.Read, System.IO.FileShare.Read);// Open the file.                dataSended = 0;                datalength =                dataToRead = iStream.Length - startBytes;// Total bytes to read:                Response.Buffer = true;                string lastUpdateTiemStr = System.IO.File.GetLastWriteTimeUtc(FilePath).ToString("r");                string eTag = HttpUtility.UrlEncode(filename, Encoding.UTF8) + lastUpdateTiemStr;                Response.AppendHeader("ETag", "\"" + eTag + "\"");//重要：续传必须                 if (startBytes > 0)                {                    if (Request.Headers["If-Range"] != null)//（IE,360）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--------------                         string If_Range = Request.Headers["If-Range"].Replace("\"", "");                        if (If_Range != eTag)                        {//文件修改过                              Response.StatusCode = 412;//预处理失败                              return "文件名验证失败";                        }                    }                    else if (Request.Headers["If-Match"] != null)//（火狐）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--------------                          string If_Match = Request.Headers["If-Match"].Replace("\"", "");                        if (If_Match != eTag)                        {//文件修改过                             Response.StatusCode = 412;//预处理失败                              return "文件名验证失败";                        }                    }                    iStream.Seek(startBytes, SeekOrigin.Begin);                    Response.AddHeader("Content-Range", string.Format(" bytes {0}-{1}/{2}", startBytes, DownloadFile.Length - 1, DownloadFile.Length));                    Response.AddHeader("Accept-Ranges", "bytes");//重要：续传必须                  }                Response.AppendHeader("Content-Length", (DownloadFile.Length - startBytes).ToString());                Response.ContentType = "application/octet-stream";                Response.AddHeader("Connection", "Keep-Alive");                Response.AddHeader("Content-Disposition", "attachment; filename=" + filename);                while (dataToRead > 0)// Read the bytes.                {                    if (Response.IsClientConnected)// Verify that the client is connected.                    {                        currentlength = iStream.Read(buffer, 0, buffersize);// Read the data in buffer.                        Response.OutputStream.Write(buffer, 0, currentlength); // Write the data to the current output stream.                        //Response.BinaryWrite(buffer);                        Response.Flush();// Flush the data to the HTML output.                        //buffer = new Byte[10000];                        dataToRead = dataToRead - currentlength;                        dataSended = dataSended + currentlength;                        Thread.Sleep(1000);//每秒钟发送一次                    }                    else                    {                        dataToRead = -1;//prevent infinite loop if user disconnects                    }                }                if (dataToRead == 0 && dataSended>0)//发送、下载完成datalength == dataSended                {                    //UpdateOrderWhenDownloaded(ResourceID, UserID, downtype);//下载完成，修改数据库状态                }                return "";            }            catch (Exception ex)            {                CommonFun.WritetTxtLog("下载出错：" + ex.Message);                return CommonFun.GetInformationHtml(webtitle, "下载出错 : " + ex.Message);            }            finally            {                if (iStream != null)                {                    iStream.Close();//Close the file.                }            }        }        #endregion        #region 返回显示标题的网页提示文本        /// 
        /// 返回显示标题的网页提示文本        /// 
        /// 网站title        /// 显示的文字        /// 
               public static string GetInformationHtml(string title, string body)        {            StringBuilder sb = new StringBuilder();            sb.Append("");            sb.Append("");            sb.AppendFormat("",title);            sb.Append("");            sb.Append("");            sb.Append("");            sb.Append("
       
        ");            sb.Append("
        
         ");            sb.Append("
         提示信息：
         
");            sb.Append(body);            sb.Append("
        
");            sb.Append("
        
         Power By 
         www.kinpan.com
         
          ");            sb.Append("
         
");            sb.Append("");            sb.Append("");            return sb.ToString() ;        }        #endregion
        
       

如果response.writefile,或mvc 中的返回 fileresult,谁知道它下载完成了，这很难控制，现在这样发送，发送完成之后就下载完成了，还能续传，比较精确。。