using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var lista = new List<string> { "1", "2", "3", "4", "5" };

        Console.WriteLine(lista.Aggregate(string.Empty, (actual, novo) => {

            if(!string.IsNullOrEmpty(actual))

                actual += ",";

            return actual + novo;           

        }));

    }

}