Clone.cs

namespace ReadonlyTest
{
    using System;
    using System.Collections.Generic;
    using System.Linq;
    using System.Text;

    public static class Program
    {
        public static void Main(string[] args)
        {
            var room = new Room();
            room.AddPersons("pesho", "gosho", "misho");
            Console.WriteLine(room);
            room.Persons.Add("ghost");
            Console.WriteLine(room);
        }
    }

    public class Room
    {
        private readonly List<string> persons = new List<string>();

        public List<string> Persons
        {
            get
            {
                return this.persons.Select(i => (string)i.Clone()).ToList();
            }
        }

        public void AddPersons(params string[] pers)
        {
            foreach (var per in pers)
            {
                this.persons.Add(per);
            }
        }

        public override string ToString()
        {
            var res = new StringBuilder();
            foreach (var person in this.persons)
            {
                res.AppendLine(person);
            }

            return res.ToString().Trim();
        }
    }
}