我在我本机上搭建HTTP网站，做了下面这个实验，结果很不理解，求各位帮忙给个说法。Stream.Read() 长度是根据什么来定，如本例中问什么是3867？GetResponseStream()返回的NetStream没有Length属性，但是不报错，为什么？

    FileStream _fsLocal = new FileStream(@"D:\HTTP\About.jpg", FileMode.Open);

    byte[] _bytesLoacal = new byte[_fsLocal.Length];

    _fsLocal.Read(_bytesLoacal, 0,Convert.ToInt32(_fsLocal.Length));

    string _path = http://XXX.XXX.X.XXX/About.jpg;

    HttpWebRequest _hwRequest = (HttpWebRequest)WebRequest.Create(_path);

    HttpWebResponse _hwResponse = (HttpWebResponse)_hwRequest.GetResponse ();

    Stream _fileStream = _hwResponse.GetResponseStream();

    Int32 _length = Convert.ToInt32(_hwResponse.ContentLength);

    byte[] bytes = new byte[_length];

     _fileStream.Read(bytes, 0, _length);

    _fileStream.Close();

    _hwResponse.Close();

            for (int i = 0; i < _length; i++)
            {

                if (_bytesLoacal[i].ToString() != bytes[i].ToString())
                {

                    MessageBox.Show(i.ToString()+"****"+_bytesLoacal[i].ToString()+"****"+bytes[i].ToString()+"\t\n");

                }

            }

            FileStream _writeFile = new FileStream("D:\\About.jpg", FileMode.Create,FileAccess.Write);

            _writeFile.Write(bytes, 0, _length);

            MessageBox.Show("下载完成");