503使用FTPS的命令顺序错误

Question

以下代码在request.GetRequestStream（）部分失败。我们收到“（503）错误的命令顺序”

var fileNameAndFtpPath = Path.Combine(ConfigurationManager.AppSettings["FtpPath"], Path.GetFileName(fileNameAndLocalPath));
byte[] fileContents;
using (StreamReader sourceStream = new StreamReader(fileNameAndLocalPath))
    fileContents = Encoding.UTF8.GetBytes(sourceStream.ReadToEnd());

FtpWebRequest request = (FtpWebRequest)WebRequest.Create(fileNameAndFtpPath);
request.KeepAlive = false;
request.UseBinary = true;
request.Method = WebRequestMethods.Ftp.UploadFile;
request.EnableSsl = true;
request.Credentials = new NetworkCredential(ConfigurationManager.AppSettings["FtpUserName"], String.Empty);
request.Proxy = new WebProxy();
request.ContentLength = fileContents.Length;

request.EnableSsl = true;
X509CertificateCollection certCollection = new X509CertificateCollection();
certCollection.Add(GetCertificate());
request.ClientCertificates = certCollection;

using (Stream requestStream = request.GetRequestStream())
    requestStream.Write(fileContents, 0, fileContents.Length);

当我们检查登录过程时，它看起来像这样：

FtpControlStream#64844482 - Sending command [USER xxx]
... (certificate stuff)
FtpControlStream#64844482 - Received response [232 User xxx logged in]
FtpControlStream#64844482 - Sending command [PASS]
FtpControlStream#64844482 - Received response [503 You are already logged in]

看起来不应该发送PASS命令，但是如何强制FtpWebRequest这样做呢？