using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.IO;
using System.Xml;
using System.Xml.Serialization;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            List<User> users;
            XmlSerializer serializer = new XmlSerializer(typeof(List<User>));

            string filename = "Users.xml";

            // Get the current user list, or initialize a new list.

            if (File.Exists(filename))
            {
                using (Stream stream = File.Open(filename, FileMode.Open))
                {
                    users = (List<User>)serializer.Deserialize(stream);
                }
            }
            else
            {
                users = new List<User>();
            }

            // Create a new User

            Console.WriteLine("Create User");

            Console.Write("Username: ");
            string username = Console.ReadLine();
            Console.Write("Password: ");
            string password = Console.ReadLine();

            users.Add(new User(username, password));

            // Save the users to the file again.

            using (Stream stream = File.Open(filename, FileMode.OpenOrCreate))
            {
                serializer.Serialize(stream, users);
            }
        }
    }

    public class User
    {
        public string Username
        {
            get;
            set;
        }

        public string Password
        {
            get;
            set;
        }

        public User()
        {
        }

        public User(string username, string password)
        {
            this.Username = username;
            this.Password = password;
        }
    }
}