Get Second to last character position from string

Question

I have a dynamically formed string like - part1.abc.part2.abc.part3.abc

In this string I want to know position of second to last \".\" so that i can split string as

Answer 1

This would be the solution, with the best possible performance (it probably won't get much faster and memory lightweight than this, unless you want to go the unsafe route):

public static int LastIndexOf(this string str, char charToSearch, int repeatCound)
{
    int index = -1;
    for(int i = str.Length - 1; i >= 0, numfound < repeatCound)
    {
        if(str[i] == charToSearch)
        {
            index = i; 
            numfound++;
        }
    }

    return index;
}