I'm trying to make a function that returns the index of the Nth occurrence of a given char in a string.

Here is my attempt:

private int IndexOfNth(string str, char c, int n)
{
    int index = str.IndexOf(c) + 1;
    if (index >= 0)
    {
        string temp = str.Substring(index, str.Length - index);
        for (int j = 1; j < n; j++)
        {
            index = temp.IndexOf(c) + 1;
            if (index < 0)
            {
                return -1;
            }
            temp = temp.Substring(index, temp.Length - index);
        }
        index = index + (str.Length);
    }
    return index;
}

This should find the first occurrence, chop off that front part of the string, find the first occurrence from the new substring, and on and on until it gets the index of the nth occurrence. However I failed to consider how the index of the final substring is going to be offset from the original actual index in the original string. How do I make this work?

Also as a side question, if I want the char to be the tab character do I pass this function '\t' or what?