nth occurence od character in a string

public int GetNthOccurenceIndex(string sample, char t, int nth)
{
    int count = 0;
    for (int i = 0; i < sample.Length; i++)
    {
        if (sample[i] == t)
        {
            count++;
            if (count == nth)
               return i;
        }
    }
    // -1 means nth occurrence was not found.
    return -1;
}