Thanks for sharing your solution.
Though I do not understand why you are even using Session.GetFileInfo. It seems ineffective.
Just use Session.ListDirectory to get entries in /SD1 (and later its subfolders), including information whether they are files or folders.
I even better, use Session.EnumerateRemoteFiles.

I work with arenn23. For context, the way we are using the .NET Assembly in this case is effectively equivalent to https://winscp.net/eng/docs/library_session_enumerateremotefiles. We have a recursive function that calls GetFileInfo to check if the current item was a file or directory, then recurse into the directories. The issue we faced was twofold:

1. The Keyence FTP server advertised support for MLSD/MLST, but appeared not to work, which is why we had to switch to LIST.
   
2. As Martin mentioned, the LIST implementation has to list the files in the parent of the current directory (in our case, /SD1). But since the Keyence FTP server we tried to interface with prevents access to the root directory /, the FTP server's access controls blocked this command from working
   

In our case, we were able to find a workaround for this issue: we simply assume the "search root" of our function is a directory and skip the GetFileInfo call on the root directory. Then, the later recursive calls only need to look at /SD1 and its subdirectories when they call LIST, which the FTP server does allow access.

Also, I just want to say thank you Martin for your support and for building such a useful WinSCP tool.

The GUI log shows listing of /SD1 folder contents. While the .NET assembly tries to list the root folder to obtain data about the SD1 folder (as without MLST, SIZE and MDTM commands, there's no other way).

If instead you actually want to list contents of the /SD1 folder in .NET assembly, use Session.ListDirectory, instead of Session.GetFileInfo.
https://winscp.net/eng/docs/library_session_listdirectory

Hi Martin,

We have been able to connect to the server using the WinSCP GUI. I will attach the debug logs for that session. I will also attach the .NET assembly logs without the raw MLSD setting. Is there an obvious reason why the GUI is successful but our script is not?

Well, then the server seem pretty much broken. It announces support for MLST, yet it then says that it doesn't know it.

WinSCP cannot provide file information, if the server has broken support for MLST command, nor supports MDTM/SIZE commands, nor listing the parent directory.

Before I set the FtpUseMlsd setting, I encountered this error:

I looked through other forums and thought you had suggested setting FtpUseMlsd to 1 in similar cases. However, I might have misunderstood. Unfortunately, I do not have detailed error logs for this initial case. Do you have any other suggestions for this issue?

I cannot tell why the server fails CWD / with "550 Unable to find /". But the reason why WinSCP even tries the CWD /, is that you disallow it from using MLST by setting FtpUseMlsd. Why are you doing that?

Hello, I am getting the following error:

[18:22:28 ERR] Failed with: System.Exception: WinSCP.SessionRemoteException: Can't get attributes of file '/SD1'.
Could not retrieve directory listing
Unable to find /

This error occurs while utilizing the .NET assembly; the GUI seems to work. I will attach the detailed logs.