[转载]续传

C# code
using System;
using System.Collections.Generic;
using System.Text;
using System.Collections.Specialized;
using System.Net;
using System.Net.Sockets;
using System.IO;
namespace CompleteClient
{
    /// <summary>
    /// 文件发送工作类
    /// </summary>
    class PostFile
    {
        /// <summary>
        /// 通过post发送指定文件的指定字段到指定的uri上
        /// </summary>
        /// <param name="uploadfile">上传文件路径</param>
        /// <param name="url">上传的到的URi位置</param>
        /// <param name="offset">当前偏移量</param>
        /// <param name="size">需要发送的块大小</param>
        /// <param name="fileFormName">服务器端"GET"取得的文件名</param>
        /// <param name="contenttype">文件类型(保留用)</param>
        /// <param name="querystring">GET数组(供服务器用GET取得一些信息)</param>
        /// <param name="cookies">本地cookies(保留用)</param>
        /// <returns>uri的response的内容以string的形式返回</returns>
        public string UploadFileEx(string uploadfile, string url, long offset, long size,
                                   string fileFormName, string contenttype,
                                   NameValueCollection querystring, CookieContainer cookies)
        {
            if ((fileFormName == null) ||
            (fileFormName.Length == 0))
            {
                fileFormName = "file";
            }

            if ((contenttype == null) ||
            (contenttype.Length == 0))
            {
                contenttype = "application/octet-stream";
            }

            string postdata;
            postdata = "?";
            if (querystring != null)
            {
                foreach (string key in querystring.Keys)
                {
                    postdata += key + "=" + querystring.Get(key) + "&";
                }
            }
            Uri uri = new Uri(url + postdata);

            string boundary = "----------" + DateTime.Now.Ticks.ToString("x");
            HttpWebRequest webrequest = (HttpWebRequest)WebRequest.Create(uri);
            webrequest.CookieContainer = cookies;
            webrequest.ContentType = "multipart/form-data; boundary=" + boundary;
            webrequest.Method = "POST";

            // 构造一个post请求的http头
            StringBuilder sb = new StringBuilder();
            sb.Append("--");
            sb.Append(boundary);
            sb.Append("\r\n");
            sb.Append("Content-Disposition: form-data; name=\"");
            sb.Append(fileFormName);
            sb.Append("\"; filename=\"");
            sb.Append(Path.GetFileName(uploadfile));
            sb.Append("\"");
            sb.Append("\r\n");
            sb.Append("Content-Type: ");
            sb.Append(contenttype);
            sb.Append("\r\n");
            sb.Append("\r\n");

            string postHeader = sb.ToString();
            byte[] postHeaderBytes = Encoding.UTF8.GetBytes(postHeader);

            // Build the trailing boundary string as a byte array
            // ensuring the boundary appears on a line by itself
            byte[] boundaryBytes =
            Encoding.ASCII.GetBytes("\r\n--" + boundary + "\r\n");

            FileStream fileStream = new FileStream(uploadfile,
                            FileMode.Open, FileAccess.Read);
            long length = postHeaderBytes.Length + (long)size +
                                       boundaryBytes.Length;
            webrequest.ContentLength = length;

            Stream requestStream = webrequest.GetRequestStream();
            // 写入post头
            requestStream.Write(postHeaderBytes, 0, postHeaderBytes.Length);

            // 写入文件内容
            byte[] buffer = new Byte[size];
            fileStream.Seek(offset, SeekOrigin.Current);
            fileStream.Read(buffer, 0, buffer.Length);

            requestStream.Write(buffer, 0, buffer.Length);

            // 写入post请求的尾
            requestStream.Write(boundaryBytes, 0, boundaryBytes.Length);
            //读取服务器的反馈消息
            WebResponse responce = webrequest.GetResponse();
            Stream s = responce.GetResponseStream();
            StreamReader sr = new StreamReader(s);

            return sr.ReadToEnd();
        }
    }
}