关于断点续传，值不在预期的范围内解决方案

关于断点续传，值不在预期的范围内
在做一个pda在线更新
下载的时候出现异常值不在预期的范围内
下面是代码
private string urlPath = "http://xx.xx.xx.xx/iss/";//根据实际需要修改
  private string Filename = "D:\\测试";
  
  public Form1()
  {
  InitializeComponent();
  }

  private void button1_Click(object sender, EventArgs e)
  {
  DownFile(urlPath + "client.exe", Filename, this.progressBar1);
  }
   
  public static void DownFile(string URL, string Filename, ProgressBar Prog)
  {
  try
  {
  HttpWebRequest Myrq = (HttpWebRequest)HttpWebRequest.Create(URL);
  HttpWebResponse myrp = (HttpWebResponse)Myrq.GetResponse();

  long totalBytes = myrp.ContentLength;
  if (Prog != null)
  {
  Prog.Maximum = (int)totalBytes;
  }

  System.IO.Stream st = myrp.GetResponseStream();
  System.IO.Stream so = new FileStream(Filename, FileMode.Create);//执行到这就进异常了
  long totalDownloadedByte = 0;
  byte[] by = new byte[1024];
  int osize = st.Read(by, 0, (int)by.Length);
  while (osize > 0)
  {
  totalDownloadedByte = osize + totalDownloadedByte;
  Application.DoEvents();
  so.Write(by, 0, osize);
  Prog.Value = (int)totalDownloadedByte;
  osize = st.Read(by, 0, (int)by.Length);
  System.Windows.Forms.Application.DoEvents(); 
  }
  so.Close();
  st.Close();
  MessageBox.Show("下载完毕!", "下载提示：", MessageBoxButtons.OK, MessageBoxIcon.Asterisk, 

MessageBoxDefaultButton.Button1);
  }

  catch (Exception ex)
  {
  Console.WriteLine(ex.ToString());
  MessageBox.Show("下载过程中出现错误:" + ex.ToString(), "系统提示");
  }
  DialogResult dr = MessageBox.Show("下载完成，是否要打开程序","提示", MessageBoxButtons.OK, 

MessageBoxIcon.Asterisk,MessageBoxDefaultButton.Button1);
  if (dr == DialogResult.OK)
  {
  System.Diagnostics.Process.Start("client.exe");
  Application.Exit();
  }
  else 
  {
  Console.WriteLine("放弃程序更新");
  }
  }

[解决办法]
参数这样
System.IO.Stream so = new FileStream(Filename, FileMode.Append, FileAccess.Write);