using System;

using System.Collections.Generic;

​

public class Program

{

    public static IEnumerable<string> GetSubStrings(string input, List<int> source)

    {

        for (var i = 0; i < source.Count; i += 2)

            yield return input.Substring(source[i], source[i + 1] - source[i]);

    }

​

    public static List<string> GetSubStrings2(string input, List<int> source)

    {

        var result = new List<string>();

        for (var i = 0; i < source.Count; i += 2)

            result.Add(input.Substring(source[i], source[i + 1] - source[i]));

        return result;

    }

​

    public static List<int> AllIndexesOf(string str, string value)

    {

        if (String.IsNullOrEmpty(value))

            throw new ArgumentException("the string to find may not be empty", "value");

        List<int> indexes = new List<int>();

        for (int index = 0;; index += value.Length)

        {

            index = str.IndexOf(value, index);

            if (index == -1)

                return indexes;

            indexes.Add(index);

        }

    }

​

    public static void Main()

    {

        var input = "2abcdef2ggg2abcdefgh2";

        var indexes = AllIndexesOf(input, "2");

        Console.WriteLine(string.Join(", ", GetSubStrings(input, indexes)));

        Console.WriteLine(string.Join(", ", GetSubStrings2(input, indexes)));

    }

}