using System;

public class Program

{

    public static void Main()

    {

        string test = "asdfasdfasdf";

        Console.WriteLine(test.NthIndexOf("a", 0));

        Console.WriteLine(test.NthIndexOf("a", 1));

        Console.WriteLine(test.NthIndexOf("a", 2));

        Console.WriteLine(test.NthIndexOf("a", 3));

        Console.WriteLine(test.NthIndexOf("a", 4));

    }

}

​

public static class StringExtensions

{

    public static int NthIndexOf(this string s, string value, int n)

    {

        var index = -1;

        for (; n > 0; n--)

            index = s.IndexOf(value, index + 1);

        return index;

    }

}