using System;

using System.Linq;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        foreach (int item in AllIndexesOf("11111", "11"))

            Console.WriteLine(item);

    }

​

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

    {

        int minIndex = str.IndexOf(searchstring);

        while (minIndex != -1)

        {

            yield return minIndex;

            minIndex = str.IndexOf(searchstring, minIndex + 1);

        }

    }

}