c＃,.net 下载文件 设置断点

        /// <summary>
        /// c#,.net 下载文件  
        /// </summary>
        /// <param name="URL">下载文件地址</param>
        /// <param name="filename">下载后的存放地址</param>
        /// <param name="prog">用于显示的进度条</param>
        /// <param name="label1"></param>
        public void DownloadFile(string URL, string filename, System.Windows.Forms.ProgressBar prog, System.Windows.Forms.Label label1)
        {
            float percent = 0;
            try
            {
                System.Net.HttpWebRequest Myrq = null, _Myrq = null;
                System.Net.HttpWebResponse myrp = null, _myrp = null;
                #region 是否存在断点并设置断点
                if (File.Exists(filename))//文件是否存在
                {
                    so = new System.IO.FileStream(filename, System.IO.FileMode.Open);//打开文件流
                    downedsize = so.Length;//已下载的大小
                    so.Seek(downedsize, SeekOrigin.Current);//设置流断点
                }
                else
                {
                    so = new System.IO.FileStream(filename, System.IO.FileMode.Create);//创建文件流
                    downedsize = 0;
                }
                #endregion
                #region 设置下载流Range值。
                _Myrq = (System.Net.HttpWebRequest)System.Net.HttpWebRequest.Create(URL);
                _myrp = (System.Net.HttpWebResponse)_Myrq.GetResponse();
                long totalBytes = _myrp.ContentLength;//文件大小值
                if (prog != null)
                {
                    prog.Maximum = (int)totalBytes;
                }
                // “基础连接已经关闭:   无法连接到远程服务器”的错误
                _myrp.Close();
                Myrq = (System.Net.HttpWebRequest)System.Net.HttpWebRequest.Create(URL);
                if (downedsize > 0 && downedsize < totalBytes)
                {
                    Myrq.AddRange((int)downedsize); //设置Range值
                }
                else if (downedsize==totalBytes)
                {
                    so.Close();
                    MessageBox.Show("您已经下载好此文件！", "提示");
                    return;
                }
                myrp = (System.Net.HttpWebResponse)Myrq.GetResponse();//剩余的流
                st = myrp.GetResponseStream();//下载的目标流
                #endregion
                long totalDownloadedByte = downedsize > 0 ? downedsize : 0;
                byte[] by = new byte[1024];
                int osize = st.Read(by, 0, (int)by.Length);
                while (osize > 0)
                {
                    totalDownloadedByte = osize + totalDownloadedByte;
                    System.Windows.Forms.Application.DoEvents();
                    so.Write(by, 0, osize);
                    if (prog != null)
                    {
                        prog.Value = (int)totalDownloadedByte;
                    }
                    osize = st.Read(by, 0, (int)by.Length);
                    percent = (float)totalDownloadedByte / (float)totalBytes * 100;
                    label1.Text = "当前补丁下载进度" + percent.ToString() + "%";
                    System.Windows.Forms.Application.DoEvents(); //必须加注这句代码，否则label1将因为循环执行太快而来不及显示信息
                }
                so.Close();
                st.Close();
                myrp.Close();//关闭流
                MessageBox.Show("下载完毕！","提示");
            }
            catch (System.Exception)
            {
            }
        }

下载例子地址：http://download.csdn.net/detail/yes16ws/4128914