如何传输大文件

这是我写的一段局域网聊天工具中传输文件的代码，可只能传输小文件，如何传输大文件，大哥们帮帮忙！
private void btnSendFile_Click(object sender, System.EventArgs e)
  {
//   Thread t = new Thread(new ThreadStart(SendFile));
//   t.Start();
   Stream s;
   OpenFileDialog d = new OpenFileDialog();
   d.Filter = "All files (*.*)|*.*" ;
   d.Multiselect = false;
   if(d.ShowDialog() == DialogResult.OK)
   {
    if((s = d.OpenFile()) != null)
    {
     FileInfo fi = new FileInfo(d.FileName);
     if(fi.Length>0)
     {
      if((fi.Length/(1024*1024))>3)
      {
       MessageBox.Show("发送文件请不要大于4M ！");
       return;
      }
      this.btnSendFile.Enabled = false;
      byte[] contents = new byte[fi.Length];
      string FileSize = (contents.Length<1049)?"0.001":Convert.ToString                                                (Convert.ToDouble(contents.Length)/(1024*1024));
      int length = (FileSize.Length>4) ? 5 : FileSize.Length;
      FileSize = FileSize.Substring(0,length);
      try
      {
       s.Read(contents, 0, contents.Length);
       ServerConst.ChatServ.SendFile(fi.Name,contents,                                                          main.userInfo.UserID,FriendInfo.UserID);
       string Message = string.Format("{0}\r\n[{1}] 向 [{2}] 发送了文件                                                          [{3}] （共{4}M），\r\n稍后请注意查收......",DateTime.Now.ToString                                                        ("[yyyy-MM-dd HH:mm:ss]"),main.userInfo.NickName,FriendInfo.NickName,fi.Name,FileSize);
       tbHistory.AppendText(Message + "\r\n\r\n");
       ServerConst.ChatServ.Send(Message,main.userInfo.UserID,FriendInfo.UserID);
      }
      catch
      {
       MessageBox.Show("发送文件失败！");
      }
      this.btnSendFile.Enabled = true;
     }
     s.Close();
    }
   }
  }