How to check if file exists on FTP before FtpWebRequest

问题

I need to use FtpWebRequest to put a file in a FTP directory. Before the upload, I would first like to know if this file exists.

What method or property should I use to check if this file exists?

回答1:

var request = (FtpWebRequest)WebRequest.Create
    ("ftp://ftp.domain.com/doesntexist.txt");
request.Credentials = new NetworkCredential("user", "pass");
request.Method = WebRequestMethods.Ftp.GetFileSize;

try
{
    FtpWebResponse response = (FtpWebResponse)request.GetResponse();
}
catch (WebException ex)
{
    FtpWebResponse response = (FtpWebResponse)ex.Response;
    if (response.StatusCode ==
        FtpStatusCode.ActionNotTakenFileUnavailable)
    {
        //Does not exist
    }
}

As a general rule it's a bad idea to use Exceptions for functionality in your code like this, however in this instance I believe it's a win for pragmatism. Calling list on the directory has the potential to be FAR more inefficient than using exceptions in this way.

If you're not, just be aware it's not good practice!

EDIT: "It works for me!"

This appears to work on most ftp servers but not all. Some servers require sending "TYPE I" before the SIZE command will work. One would have thought that the problem should be solved as follows:

request.UseBinary = true;

Unfortunately it is a by design limitation (big fat bug!) that unless FtpWebRequest is either downloading or uploading a file it will NOT send "TYPE I". See discussion and Microsoft response here.

I'd recommend using the following WebRequestMethod instead, this works for me on all servers I tested, even ones which would not return a file size.

WebRequestMethods.Ftp.GetDateTimestamp

回答2:

Because

request.Method = WebRequestMethods.Ftp.GetFileSize

may fails in some case (550: SIZE not allowed in ASCII mode), you can just check Timestamp instead.

reqFTP.Credentials = new NetworkCredential(inf.LogOn, inf.Password);
reqFTP.UseBinary = true;
reqFTP.Method = WebRequestMethods.Ftp.GetDateTimestamp;

回答3:

FtpWebRequest (nor any other class in .NET) does not have any explicit method to check a file existence. You need to abuse a request like GetFileSize or GetDateTimestamp.

string url = "ftp://ftp.example.com/remote/path/file.txt";

WebRequest request = WebRequest.Create(url);
request.Credentials = new NetworkCredential("username", "password");
request.Method = WebRequestMethods.Ftp.GetFileSize;
try
{
    request.GetResponse();
    Console.WriteLine("Exists");
}
catch (WebException e)
{
    FtpWebResponse response = (FtpWebResponse)e.Response;
    if (response.StatusCode == FtpStatusCode.ActionNotTakenFileUnavailable)
    {
        Console.WriteLine("Does not exist");
    }
    else
    {
        Console.WriteLine("Error: " + e.Message);
    }
}

If you want a more straightforward code, use some 3rd party FTP library.

For example with WinSCP .NET assembly, you can use its Session.FileExists method:

SessionOptions sessionOptions = new SessionOptions {
    Protocol = Protocol.Ftp,
    HostName = "ftp.example.com",
    UserName = "username",
    Password = "password",
};

Session session = new Session();
session.Open(sessionOptions);

if (session.FileExists("/remote/path/file.txt"))
{
    Console.WriteLine("Exists");
}
else
{
    Console.WriteLine("Does not exist");
}

(I'm the author of WinSCP)

回答4:

I use FTPStatusCode.FileActionOK to check if file exists...

then, in the "else" section, return false.

来源：https://stackoverflow.com/questions/347897/how-to-check-if-file-exists-on-ftp-before-ftpwebrequest

标签

.net

ftp

ftpwebrequest