static void Main()
        {
            List<String> str = new List<string>() { "a", "b", "c" };

            Method(str);
            foreach (var item in str)
            {
                Console.WriteLine(item);
            }
            Method1(ref str);

            foreach (var item in str)
            {
                Console.WriteLine(item);
            }

            Console.ReadLine();
        }

        private static void Method1(ref List<string> str)
        {
            str.Add("d");
        }

        private static void Method(List<string> str)
        {
            str = new List<string>() { "1", "2", "3" };
        }