Tcp C# 数据过多造成数据丢失，求助

if (!isFailed)
            {
                try
                {
                    byte[] buffer = new byte[1024 * 1024];
                    int totalLengh = 0;
                    do
                    {

                        int headLen = stream.Read(buffer, totalLengh, buffer.Length - totalLengh);
                        totalLengh += headLen;
                        if (totalLengh == buffer.Length)
                        {
                          string datagram = Encoding.UTF8.GetString(buffer, 0, totalLengh);  
                        }
                        if (headLen == 0)
                        {
                            //finished
                            errorMessage = "远程服务器已关闭，无法接收数据";
                            isFailed = true;
                            break;
                        } // 读数据包体本身
数据大于1M，datagram没有完全接收数据，咋办？