我服务器上文件业务上只能下载一次，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
/// <summary>
/// 下载文件处理
/// </summary>
/// <param name="ResourceID">资源id</param>
/// <param name="UserID">用户id</param>
/// <param name="downtype">下载类型：1-购买，2-今日免费，3-订阅用户</param>
/// <param name="webtitle">提示页面titie</param>
/// <param name="ResourceType">资源类型（暂时不考虑）</param>
/// <returns></returns>
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,&nb