public void Worker()
        {
            string url = "https://files.cnblogs.com/cnryb/HttpSer.zip";
            string path = "a.zip";
            
            using (FileStream fs = new FileStream(
                path,
                FileMode.OpenOrCreate,
                FileAccess.Write,
                FileShare.Read,
                2048,
                FileOptions.RandomAccess
                ))
            {
                //先下载文件的第100个字节到最后
                fs.Seek(100, SeekOrigin.Begin);
                Down(fs, url, 100);

                //下载前100个字节
                fs.Seek(0, SeekOrigin.Begin);
                Down(fs, url, 0, 99);
            }
        }

public void Down(FileStream fs, string url, int start, int end = -1)
        {
            HttpWebRequest hwr = WebRequest.Create(url) as HttpWebRequest;
            if (end == -1)
                hwr.AddRange(start);
            else
                hwr.AddRange(start, end);
            HttpWebResponse resp = hwr.GetResponse() as HttpWebResponse;
            using (Stream input = resp.GetResponseStream())
            {
                byte[] bs = new byte[2048];
                int count;
                while ((count = input.Read(bs, 0, bs.Length)) != 0)
                {
                    fs.Write(bs, 0, count);
                }
            }
        }