HttpWebRequestでCookieを使う

Cookie.cs

private static CookieContainer cc = new CookieContainer();
public static byte[] GetStream(string url)
{
  var req = HttpWebRequest.CreateHttp(url);
  req.CookieContainer = cc;

  using (var res = req.GetResponse())
  using (var sr = res.GetResponseStream())
  using (var sw = new MemoryStream())
  {
    var totalBytes = 0;
    var readData = new byte[1024];

    while(true)
    {
      var readSize = sr.Read(readData, 0, readData.Length);
      if (readSize == 0)
      {
        break;
      }

      totalBytes += readSize;

      sw.Write(readData, 0, readSize);
    }

    return sw.ToArray();
  }
}