using System;

using System.Linq;

using System.Text;

​

public class Program

{

    public static void Main()

    {

        var chars = new[]

        {

        "a", "b", "c", "d"

        }

​

        ;

        var csv = chars.Aggregate(new StringBuilder(), (a, b) =>

            {

                if (a.Length > 0)

                    a.Append(",");

                a.Append(b);

                return a;

            }

​

        );

        Console.WriteLine(csv);

    }

}