using System;

using System.Linq;

using System.Collections.Generic;

using System.Collections;

​

static class ExtensionMethods

{

    static public IEnumerable<char> GetDelimitedField(this IEnumerable<char> source, char delimiter, int index)

    {

        foreach (var c in source)

        {

            if (c == delimiter) 

            {

                if (--index < 0) yield break;

            }

            else

            {

                if (index == 0) yield return c;

            }

        }

    }

​

    static public string GetDelimitedString(this string source, char delimiter, int index)

    {

        var result = source.GetDelimitedField(delimiter, index);

        return new string(result.ToArray());

    }

}

​

public class Program

{

    public static void Main()

    {

        var input = "Zero,One,Two,Three,Four,Five,Six";

        var output = input.GetDelimitedString(',',5);

        Console.WriteLine(output);

    }

}