OpenAppend.cs

using System;
using System.IO;
using System.Net;
using FluentFTP;

namespace Examples {
    public class OpenAppendExample {
        public static void OpenAppend() {
            using (FtpClient conn = new FtpClient()) {
                conn.Host = "localhost";
                conn.Credentials = new NetworkCredential("ftptest", "ftptest");
                
                using (Stream ostream = conn.OpenAppend("/full/or/relative/path/to/file")) {
                    try {
                        // be sure to seek your output stream to the appropriate location, i.e., istream.Position
                        // istream.Position is incremented accordingly to the writes you perform
                        // istream.Position == file size if the server supports getting the file size
                        // also note that file size for the same file can vary between ASCII and Binary
                        // modes and some servers won't even give a file size for ASCII files! It is
                        // recommended that you stick with Binary and worry about character encodings
                        // on your end of the connection.
                    }
                    finally {
                        ostream.Close();
                    }
                }
            }
        }
    }
}