Because you might want to use this code throughout your project it is implemented as an Extension class. Simply put: the IndexOfAll method is attached to the String class. So if we want to call it we can just use .IndexOfAll(needle)

To be able to define an extension we need to create a static method and put it into a static class. The first parameter of the method identifies the class the method should associate with. In our case: string. We do this by defining it as “this string”.

using System;
using System.Collections;

namespace StringItems
{
    static class StringExt
    {
        public static IEnumerable IndexOfAll(this string haystack, string needle)
        {
            int pos,offset = 0;
            while ((pos = haystack.IndexOf(needle))>0)
            {
                haystack = haystack.Substring(pos+needle.Length);
                offset += pos;
                yield return offset;
            }
        }
    }

    class MainClass
    {
        public static void Main(string[] args)
        {
            string needle = "x";
            string haystack = "3 x 4 = 2 x 6 = 1 x 12";
            foreach(int Pos in haystack.IndexOfAll(needle))
                Console.WriteLine("Offset: {0}",Pos);
        }
    }
}

This is a post from Martijn's C# Coding Blog.