using System;

using System.Linq;

using System.Collections.Generic;

​

public static class Program

{

    public static void Main()

    {

        string s = "A lot, of text, with commas, here and,there";

        int[] result = s.AllIndexesOf(",").ToArray();

        foreach (int index in result)

        {

            Console.Write(index + "\t");

        }

    }

​

    public static IEnumerable<int> AllIndexesOf(this string str, string searchstring)

    {

        int minIndex = str.IndexOf(searchstring);

        while (minIndex != -1)

        {

            yield return minIndex;

            minIndex = str.IndexOf(searchstring, minIndex + searchstring.Length);

        }

    }

}