会话丢失，无法访问下载数据

public static byte[] BBStorageGetFile(string filePath, int projectId) 
{
    var queryString = QueryStringEncoder.bbs_encode("a=g&p=" + filePath + "&s=" + projectId);
    var url = "http://www.buildbinder.com/base/storageFile.aspx?" + queryString;
    string urlB = HttpContext.Current.Request.Url.ToString();
    if (urlB.Contains("intranet.buildbinder.com"))
    {
        url = "http://intranet.buildbinder.com:8000/base/storageFile.aspx?" + queryString; 
    }
    Uri uri = new Uri(url);
    WebClient webClient = new WebClient();
    webClient.Credentials = CredentialCache.DefaultCredentials;
    webClient.UseDefaultCredentials = true;
    webClient.Headers.Add(HttpRequestHeader.Cookie, "culture");
    webClient.Headers.Add(HttpRequestHeader.Cookie, "mySession");
    webClient.Headers.Add(HttpRequestHeader.ContentType, "application/x-www-form-urlencoded");
    webClient.Headers.Add(HttpRequestHeader.UserAgent, "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.2; .NET CLR 1.0.3705;)");
    byte[] file = webClient.DownloadData(uri);
    return file;
}