错误，

  while (true)
                {
                    byte[] startMsg = new byte[5];
                    while (client.Receive(startMsg) > 0)//有问题，不一定返回5
                    {
                        string msg = Encoding.ASCII.GetString(startMsg);
                        if (msg.EndsWith("<^F^>"))
                            break;
                    }
                    byte[] bt = new byte[4];
                    int avNum = 0;
                    int count = 0;
                    int sizeOnce = 1024;
                    client.Receive(bt);//不一定读取4个字节，查看MSDN
                    int lenght = BitConverter.ToInt32(bt, 0);
                    byte[] bit = new byte[lenght];
                    while ((avNum = client.Receive(bit, count, sizeOnce, SocketFlags.None)) > 0)
                    {
                        count = count + avNum;
                        if (count + sizeOnce < lenght)
                        {
                            sizeOnce = 1024;
                        }
                        else
                        {
                            sizeOnce = lenght - count;
                            break;
                        }

                    }

//MSDN

如果当前使用的是面向连接的 Socket，那么 Receive 方法将会读取所有可用的数据，直到达到缓冲区的大小为止。

意思是如果协议栈中可用数据大于buff那么读完buff大小返回，如果协议栈暂时有的数据小于buff读完协议栈返回。

如果面向无连接的，UDP， 协议栈大小》buff, 读完buff其余的丢弃