public static class TcpClientEx {
        public static bool IsOnline(this TcpClient c) {
            return !((c.Client.Poll(1000, SelectMode.SelectRead) && (c.Client.Available == 0)) || !c.Client.Connected);
        }
    } 

var conn=state as TcpClient;
while(conn.IsOnline()){//当网络连接未中断时循环
    using (var s = conn.GetStream()){
            var buff=new byte[512];
            if(s.DataAvailable){ //判断有数据再读，否则Read会阻塞线程。后面的业务逻辑无法处理
                var len = s.Read(buff,0,buff.Length);
            }
            //略...
    }
}