How to get the Index of second comma in a string

You have to use code like this.

int index = s.IndexOf(',', s.IndexOf(',') + 1);

You may need to make sure you do not go outside the bounds of the string though. I will leave that part up to you.

I just wrote this Extension method, so you can get the nth index of any sub-string in a string.

Note: To get the index of the first instance, use nth = 0.

public static class Extensions
{
    public static int IndexOfNth(this string str, string value, int nth = 0)
    {
        if (nth < 0)
            throw new ArgumentException("Can not find a negative index of substring in string. Must start with 0");
        
        int offset = str.IndexOf(value);
        for (int i = 0; i < nth; i++)
        {
            if (offset == -1) return -1;
            offset = str.IndexOf(value, offset + 1);
        }
        
        return offset;
    }
}