using System;

using System.Collections.Generic;

using System.Text;

public class Program

{

    public static void Main()

    {

        var strings = new List<string> { "Hello", "World" };

        Console.WriteLine(Join(strings));

        Console.WriteLine(string.Join(", ", strings));

    }

    public static String Join(List<String> strings)

        {

            StringBuilder result = new StringBuilder();

            Boolean first = true;

            foreach (String s in strings)

            {

                if (!first) result.Append(", ");

​

                result.Append(s);

                first = false;

            }

​

            return result.ToString();

        }

}