关于断点续传

下面的程式为什么不能支持断点续传？AddRange为何无效果？指指点，程序如下using System;public class ProgramClass
{
    const int iBytes = 4096; 
    static void Main(string[] args)
    {        string StrFileName = @"c:\xxx.xxx";
        string StrUrl = @"http://xxx.xxx.xxx/xxx.xxx";         //打开上次下载的文件或新建文件
        long lStartPos = 0;
        System.IO.FileStream fs;
        if (System.IO.File.Exists(StrFileName))
        {
            fs = System.IO.File.OpenWrite(StrFileName);
            lStartPos = fs.Length;
            fs.Seek(lStartPos, System.IO.SeekOrigin.Current); //移动文件流中的当前指针
        }
        else
        {
            fs = new System.IO.FileStream(StrFileName, System.IO.FileMode.Create);
            lStartPos = 0;
        }        //打开网络连接
        try
        {
            System.Net.HttpWebRequest request = (System.Net.HttpWebRequest)System.Net.HttpWebRequest.Create(StrUrl);
            System.Net.HttpWebResponse rp;
            rp = (System.Net.HttpWebResponse)request.GetResponse();
            long itotalsendlength = rp.ContentLength;
            if (lStartPos >= itotalsendlength)
            {
                Console.WriteLine("文件已下载！");
                return;
            }
            if (lStartPos > 0)
                request.AddRange((int)lStartPos); //设置Range值            rp = (System.Net.HttpWebResponse)request.GetResponse();
            long tl = rp.ContentLength;            //向服务器请求，获得服务器回应数据流
            System.IO.Stream ns = rp.GetResponseStream();            byte[] nbytes = new byte[iBytes];
            int nReadSize = 0;
            long ntotal = fs.Length;
            nReadSize = ns.Read(nbytes, 0, iBytes);
            while (nReadSize > 0)
            {
                ntotal += nReadSize;
                fs.Write(nbytes, 0, nReadSize);
                 Console.WriteLine("已下载字节数：" + ntotal);
                nReadSize = ns.Read(nbytes, 0, iBytes);
            }
            fs.Close();
            ns.Close();
            Console.WriteLine("下载完成");
        }
        catch (Exception ex)
        {
            fs.Close();
            Console.WriteLine("下载过程中出现错误:" + ex.ToString());
        }
    }}