utf-16 aware substring replace

        public static string ReplaceSubstring(this string s, string replacement, int startIndex, int length)
        {
            if (s == null)
                throw new ArgumentNullException("s", "Input string cannot be null");
            else if (startIndex < 0)
                throw new ArgumentOutOfRangeException("startIndex", "startIndex cannot be negative");
            else if (length < 0)
                throw new ArgumentOutOfRangeException("length", "length cannot be negative");

            var endIndex = startIndex + length;
            int index = 0, actualFrom = 0, actualTo = 0;
            var foundFrom = false;
            foreach (var c in s)
            {
                if (index == endIndex)
                    break;
                else if (index == startIndex)
                    foundFrom = true;

                if (!char.IsHighSurrogate(c))
                    index++;

                if (!foundFrom)
                    actualFrom++;

                actualTo++;
            }

            if (startIndex > index)
                throw new ArgumentOutOfRangeException("startIndex", $"\"startIndex\" argument is outside string boundaries: {startIndex} > {index}");
            else if (endIndex > index)
                throw new ArgumentOutOfRangeException("length", $"\"length\" argument is outside string boundaries: {endIndex} > {index}");

            return string.Concat(
                s.Substring(0, actualFrom),
                replacement,
                s.Substring(actualTo));
        }