文件下载问题，获取登录页面而不是实际文件

Question

尝试使用HttpWebRequest和响应对象下载文件。 响应不是实际文件，而是返回登录屏幕的HTML。

这是我的代码：

int bytesToRead = 10000;

// Buffer to read bytes in chunk size specified above
byte[] buffer = new Byte[bytesToRead];
string Url = "https://myurl/filename.pdf";
System.Net.ServicePointManager.ServerCertificateValidationCallback = delegate { return true; };
HttpWebRequest fileReq = (HttpWebRequest)HttpWebRequest.Create(Url);
fileReq.Timeout = 1000000;
fileReq.KeepAlive = true;

string userID= "emailid";
string Password= "password";



//string credentials = "emailid:password";
//fileReq.Headers.Add("Authorization", "Basic " + Convert.ToBase64String(Encoding.UTF8.GetBytes(credentials)));

fileReq.PreAuthenticate = true;

fileReq.Headers.Add("user", userID);
fileReq.Headers.Add("pass", Password);

//CredentialCache wrCache =
//              new CredentialCache();
//wrCache.Add(new Uri(Url), "Basic",
//  new NetworkCredential(userID, Password));
//fileReq.Credentials = wrCache;

//Create a response for this request
HttpWebResponse fileResp = (HttpWebResponse)fileReq.GetResponse();              



if (fileReq.ContentLength > 0)
    fileResp.ContentLength = fileReq.ContentLength;                


string resultString = string.Empty;
using (Stream rstream = fileResp.GetResponseStream())
{
    using (StreamReader sr = new StreamReader(rstream))
    {
        rstream.Flush();
        resultString = sr.ReadToEnd();
    }
}

resultString，是pdf文件所在网站的登录页面的HTML。

我已经尝试了几种方法（在上面的代码中注释）来传递用户ID和密码，没有工作。

我可以使用具有相同用户ID和密码的浏览器中的URL直接下载文件