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In attempting to download a file from a remote FTP server, the following exception is thrown:

The underlying connection was closed: The server committed a protocol violation.

The file was previously downloaded via a WebClient class but I am trying to implement a new solution with a FtpWebRequest per this MSFT example.

This error throws when calling request.GetResponse() below:

string remoteFilePath = 

    ConfigWrapper.GetEncryptedString(

        "FtpFeedPath", 

         @"ftp://subdomain.domain.com/folder/subfolder/file.xml.gz");

string tempFilePath = Path.GetTempPath() + @"ftpFile.gz";

string fileExtractName = Path.GetTempPath() + @"ftpFileftpFile.xml";



if (!Directory.Exists(Path.GetDirectoryName(fileExtractName)))

{

    Directory.CreateDirectory(Path.GetDirectoryName(fileExtractName));

}



FtpWebRequest request = 

    (FtpWebRequest)WebRequest.Create(remoteFilePath);

request.Method = WebRequestMethods.Ftp.DownloadFile;



request.Timeout = -1;

request.UseBinary = true;

request.KeepAlive = true;

request.UsePassive = false;





request.Credentials = new NetworkCredential(

    ConfigWrapper.GetEncryptedString(

        "FtpUserName", 

        "123456"),

    ConfigWrapper.GetEncryptedString(

        "FtpPassword", 

        "password"),

    remoteFilePath); 



FtpWebResponse response = (FtpWebResponse)request.GetResponse();



Stream responseStream = response.GetResponseStream();

StreamReader reader = new StreamReader(responseStream);

Console.WriteLine(reader.ReadToEnd());

Console.WriteLine($"Download Complete, status{response.StatusDescription}");



reader.Close();

response.Close();

Digging into the exception shows this message:
ContentType = '((System.Net.WebException)ex).Response.ContentType' threw an exception of type 'System.NotImplementedException'

Most of the other StackOverflow answers related to this issue seem to resolve after changing the KeepAlive property. That's not the case here. The other common recommendation is to get a network trace, but most posts have not discussed the issue or retrieved a trace. The trace is below.

One thing that sticks out from the trace is that the USER FTP command includes the full file path instead of just the server username. Is there a way to first establish authentication with the server before requesting a download with the full path? The MSFT example seems to specify the whole path right away.

System.Net Information: 0 : [25984] RAS supported: True

System.Net Information: 0 : [25984] FtpControlStream#43231651 - Created         

   connection from XX.XX.XX.XX:65106 to XX.XX.XX.XX:21.

System.Net Information: 0 : [25984] Associating FtpWebRequest#58529038 with     FtpControlStream#43231651

System.Net Information: 0 : [25984] FtpControlStream#43231651 - Received response [220 Remote FTP Server.  All transfers are logged.]

System.Net Information: 0 : [25984] FtpControlStream#43231651 - Sending command [USER ftp://subdomain.domain.com/folder/subfolder/file.xml.gz1850771]

System.Net Information: 0 : [25984] FtpWebRequest#58529038::(Releasing FTP connection#43231651.)

System.Net Error: 0 : [25984] Exception in FtpWebRequest#58529038::GetResponse - The underlying connection was closed: The server committed a protocol violation..

   at System.Net.FtpWebRequest.SyncRequestCallback(Object obj)

   at System.Net.FtpWebRequest.RequestCallback(Object obj)

   at System.Net.CommandStream.Dispose(Boolean disposing)

   at System.IO.Stream.Close()

   at System.IO.Stream.Dispose()

   at System.Net.ConnectionPool.Destroy(PooledStream pooledStream)

   at System.Net.ConnectionPool.PutConnection(PooledStream pooledStream, Object owningObject, Int32 creationTimeout, Boolean canReuse)

   at System.Net.FtpWebRequest.FinishRequestStage(RequestStage stage)

   at System.Net.FtpWebRequest.GetResponse()

One thing that sticks out from the trace is that the USER FTP command includes the full file path instead of just the server username.

I believe that you cause that yourself by passing an URL (remoteFilePath) to domain parameter of NetworkCredential:

request.Credentials = new NetworkCredential(

    ConfigWrapper.GetEncryptedString(

        "FtpUserName", 

        "123456"),

    ConfigWrapper.GetEncryptedString(

        "FtpPassword", 

        "password"),

    remoteFilePath); 

That makes no sense. If you need domain, pass domain, not URL. Though in most cases, domain is used with IIS only, what does not seem to be your case. So usually, you use NetworkCredential constructor overload with two arguments only (username and password).