iDomdom

private class Range
        {
            public int start;
            public int end;
        }

        public class Product
        {
            public string number;
            public int smth;
        }

        public static object Convert(ICollection<Product> value)
        {
            List<int> productsNumbers = new List<int>();
            string numbers = "";

            if (value != null)
            {
                var orderedProdCollection = value.OrderBy(p => int.Parse(p.number));
                foreach (var p in orderedProdCollection)
                    productsNumbers.Add(int.Parse(p.number));
            }
            else return numbers;

            List<Range> ranges = new List<Range>();
            Range range = new Range { start = productsNumbers[0], end = productsNumbers[0] };
            int current;

            if (productsNumbers.Count > 1)
            {
                for (int i = 1; i < productsNumbers.Count; i++)
                {
                    current = productsNumbers[i];

                    if (current == range.end + 1)
                        range.end = current;

                    if (current > range.end + 1)
                    {
                        ranges.Add(range);
                        range = new Range() { start = current, end = current };
                    }
                    if (i == productsNumbers.Count-1)
                            ranges.Add(range);
                }
                foreach (var r in ranges)
                {
                    if (r.start == r.end)
                        numbers += r.start.ToString() + ", ";
                    if ((r.end - r.start) == 1)
                        numbers += r.start.ToString() + ", " + r.end.ToString() + ", ";
                    if ((r.end - r.start) > 1)
                        numbers += r.start.ToString() + "-" + r.end.ToString() + ", ";
                }
                numbers = numbers.Substring(0, numbers.Length - 2);
            }
            else numbers = productsNumbers[0].ToString();
            return numbers;
        }
        public static object ConvertBack(string value)
        {
            List<string> nums = new List<string>();
            string nonSpacesValue = value.Replace(" ", "");
            string[] numsArr = nonSpacesValue.Split(new char[] { ',' }, StringSplitOptions.RemoveEmptyEntries);
            char dash = '-';

            foreach (string n in numsArr)
            {
                int d = n.IndexOf(dash);
                if (d == -1)
                    nums.Add(n);
                else
                {
                    int start = int.Parse(n.Substring(0, d));
                    int end = int.Parse(n[(d + 1)..]);
                    for (int i=start; i <=end ; i++)
                        nums.Add(i.ToString());
                }
            }
            return nums;