Get the index of the nth occurrence of a string?

You really could use the regular expression /((s).*?){n}/ to search for n-th occurrence of substring s.

In C# it might look like this:

public static class StringExtender
{
    public static int NthIndexOf(this string target, string value, int n)
    {
        Match m = Regex.Match(target, "((" + Regex.Escape(value) + ").*?){" + n + "}");

        if (m.Success)
            return m.Groups[2].Captures[n - 1].Index;
        else
            return -1;
    }
}

Note: I have added Regex.Escape to original solution to allow searching characters which have special meaning to regex engine.

That's basically what you need to do - or at least, it's the easiest solution. All you'd be "wasting" is the cost of n method invocations - you won't actually be checking any case twice, if you think about it. (IndexOf will return as soon as it finds the match, and you'll keep going from where it left off.)

After some benchmarking, this seems to be the simplest and most effcient solution

public static int IndexOfNthSB(string input,
             char value, int startIndex, int nth)
        {
            if (nth < 1)
                throw new NotSupportedException("Param 'nth' must be greater than 0!");
            var nResult = 0;
            for (int i = startIndex; i < input.Length; i++)
            {
                if (input[i] == value)
                    nResult++;
                if (nResult == nth)
                    return i;
            }
            return -1;
        }

System.ValueTuple ftw:

var index = line.Select((x, i) => (x, i)).Where(x => x.Item1 == '"').ElementAt(5).Item2;

writing a function from that is homework

Or something like this with the do while loop

 private static int OrdinalIndexOf(string str, string substr, int n)
    {
        int pos = -1;
        do
        {
            pos = str.IndexOf(substr, pos + 1);
        } while (n-- > 0 && pos != -1);
        return pos;
    }

Tod's answer can be simplified somewhat.

using System;

static class MainClass {
    private static int IndexOfNth(this string target, string substring,
                                       int seqNr, int startIdx = 0)
    {
        if (seqNr < 1)
        {
            throw new IndexOutOfRangeException("Parameter 'nth' must be greater than 0.");
        }

        var idx = target.IndexOf(substring, startIdx);

        if (idx < 0 || seqNr == 1) { return idx; }

        return target.IndexOfNth(substring, --seqNr, ++idx); // skip
    }

    static void Main () {
        Console.WriteLine ("abcbcbcd".IndexOfNth("bc", 1));
        Console.WriteLine ("abcbcbcd".IndexOfNth("bc", 2));
        Console.WriteLine ("abcbcbcd".IndexOfNth("bc", 3));
        Console.WriteLine ("abcbcbcd".IndexOfNth("bc", 4));
    }
}

Output

1
3
5
-1