Extension method for the System.String class, that reports the index of the N-th occurence of a specified string within the current System.String object.

StringExtensions.cs

namespace System
{
    public static class StringExtensions
    {
        /// <summary>
        ///     Reports the index of the N-th occurence of a specified string within the current System.String object.
        /// </summary>
        /// <param name="source">Source string.</param>
        /// <param name="value">The string to seek.</param>
        /// <param name="n">
        ///     Zero-based index, representing N-th occurrence of a specified string within the current System.String
        ///     object.
        /// </param>
        /// <returns></returns>
        public static int NthIndexOf(this string source, string value, int n)
        {
            int result = 0;
            while (true)
            {
                int index = source.IndexOf(value, StringComparison.Ordinal);
                if (index < 0)
                {
                    return index;
                }
                result += index;
                if (n == 0)
                {
                    return result;
                }
                result += value.Length;
                source = source.Substring(index + value.Length);
                n--;
            }
        }
    }
}