class Program
    {

        static void Main(string[] args)
        {
            var list = new List<int>() {1, 2, 3};
            var list2 = new List<int>();
            list.CopyTo(list2);
        }

    }

    public static class ListExtension
    {
        public static void CopyTo(this List<int> list, List<int> result)
        {
            foreach (var element in list)
                result.Add(element);
        }
    }