using System;

using System.Collections.Generic;

​

public static class StringListExtensions{

    public static string Join(this List<string> stringList, string separator){

        return string.Join(separator, stringList);

    }

}

public class Program

{

    public static void Main()

    {

        Console.WriteLine(new List<string>{"a", "b"}.Join(","));

    }

}