public void Worker()

        {

            string url = "http://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);

                }

            }

        }