using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var list = new string[] {"A", "B", "C"};

        var output = list.Join(",");

        Console.WriteLine(output);

    }

}

​

public static class MyExtensions {

    public static string Join(this IEnumerable<string> arr, string seperator) =>

        string.Join(seperator, arr);

}