实现断点续传下载

             string StrFileName = "c://123.zip"; //   定义文件名称
                string StrUrl = "http://pan.baidu.com//123.zip"; //  定义网络服务器地址

                //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);
                    if (lStartPos > 0)
                        request.AddRange((int)lStartPos); //设置Range值    

                    //向服务器请求，获得服务器回应数据流    
                    System.IO.Stream ns = request.GetResponse().GetResponseStream();

                    byte[] nbytes = new byte[512];
                    int nReadSize = 0;
                    nReadSize = ns.Read(nbytes, 0, 512);
                    while (nReadSize > 0)
                    {
                        fs.Write(nbytes, 0, nReadSize);
                        nReadSize = ns.Read(nbytes, 0, 512);
                    }
                    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());
                }