FileSystemWatcher for FTP

You're going to have to implement a polling solution, where you keep asking for the directory content periodically. Compare this to a cached list from the previous call and determine what happened that way.

There's nothing in the FTP protocol that will help you with this unfortunately.

The FileSystemWatcher class works by registering for events with the host Windows operating system. As such, it is limited to working on local paths and UNC paths to directories hosted on Windows systems. The MSDN documentation on FileSystemWatcher explains the paths which you can use and some of the potential problems with using the class.

If you are looking to be alerted to changes on an FTP site, you will have to use a polling mechanism to ask for the current status of files or folders you are interested in monitoring. You will be able to see when files are added and removed by comparing snapshots of the FTP site for changes and raising similar events when you detect changes. Unfortunately you wont be able to detect rename events, but other changes should be simple to monitor this way.

Write a simple service to create FileSystemWatcher, pointing at your ftp location.

Then when a file is uploaded or modified, an event will be fired in your service, which you can then use to copy the file to your local machine.
File.Copy etc.

Hav a look at: this blog

You cannot use the FileSystemWatcher or any other way, because the FTP protocol does not have any API to notify a client about changes in the remote directory.

All you can do is to periodically iterate the remote tree and find changes.

It's actually rather easy to implement, if you use an FTP client library that supports recursive listing of a remote tree. Unfortunately, the built-in .NET FTP client, the FtpWebRequest does not. But for example with WinSCP .NET assembly, you can use the Session.EnumerateRemoteFiles method.

See the article Watching for changes in SFTP/FTP server:

// Setup session options
SessionOptions sessionOptions = new SessionOptions
{
    Protocol = Protocol.Ftp,
    HostName = "example.com",
    UserName = "user",
    Password = "password",
};

using (Session session = new Session())
{
    // Connect
    session.Open(sessionOptions);

    List<string> prevFiles = null;

    while (true)
    {
        // Collect file list
        List<string> files =
            session.EnumerateRemoteFiles(
                "/remote/path", "*.*", EnumerationOptions.AllDirectories)
            .Select(fileInfo => fileInfo.FullName)
            .ToList();
        if (prevFiles == null)
        {
            // In the first round, just print number of files found
            Console.WriteLine("Found {0} files", files.Count);
        }
        else
        {
            // Then look for differences against the previous list
            IEnumerable<string> added = files.Except(prevFiles);
            if (added.Any())
            {
                Console.WriteLine("Added files:");
                foreach (string path in added)
                {
                    Console.WriteLine(path);
                }
            }

            IEnumerable<string> removed = prevFiles.Except(files);
            if (removed.Any())
            {
                Console.WriteLine("Removed files:");
                foreach (string path in removed)
                {
                    Console.WriteLine(path);
                }
            }
        }

        prevFiles = files;

        Console.WriteLine("Sleeping 10s...");
        Thread.Sleep(10000);
    }
}

(I'm the author of WinSCP)

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Though, if you actually want to just download the changes, it's a way easier. Just use the Session.SynchronizeDirectories in the loop.

session.SynchronizeDirectories(
    SynchronizationMode.Local, "/remote/path", @"C:\local\path", true).Check();

See the article Keep local directory up to date (download changed files from remote SFTP/FTP server).

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If you do not want to use a 3rd party library, you have to do with limitations of the FtpWebRequest. For an example how to recursively list a remote directory tree with the FtpWebRequest, see my answer to C# Download all files and subdirectories through FTP.