using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var myList = new List<string> { "1", "3-6", "2", "8-10", "7", "11", "13-18", "12" };

        var ranges = myList.Select(x=> new Range(x)).ToList();

        ranges.Sort();//Order yourself!

​

        //Display

        foreach (var range in ranges) Console.WriteLine(range);

​

        //One line with comma separator

        Console.WriteLine(string.Join(", ", ranges));

    }

    public class Range : IComparable<Range>

    {

        // Simple ctor, with Customisable separator in case of Negative Value. 

        public Range(string input, char separator= '-') {

            if (!input.Contains(separator))// Is it a range?

            {//It's not then

                if (int.TryParse(input, out int value))// Lets try to convert it to an integer

                {// it's a int then

                    Min = value; Max = value; // both min and max 

                }

                else

                {// It's not a range and it's not an int! Scream!

                    throw new ArgumentException(input + "  is not a valid input !");

                }

            }

            else 

            {//It's a range

                var parts = input.Split(new char[] { separator }, 2);// split in 2 on the separator

                if (int.TryParse(parts[0], out int part1) && int.TryParse(parts[1], out int part2))// Both part are int? 

                {                        

                    Min = Math.Min(part1, part2); // min is the min and max the max.

                    Max = Math.Max(part1, part2); // It's easier to compute min/max than try to fix a bad input.

                }

                else {// It's look like a range but those are not int! Scream!

                    throw new ArgumentException(input + "  is not a valid input !" );

                }

            }

        }

        public int Min { get; set; }

        public int Max { get; set; }

​

        // Comparing bound together. {1-1} < {1-2} < {1-6} < {2-2}; 

        public int CompareTo(Range obj)

        {

            var mincomparer = this.Min.CompareTo(obj.Min);

            var maxcomparer = this.Max.CompareTo(obj.Max);

            return mincomparer == 0 ? maxcomparer : mincomparer;

        }

​

        public override string ToString()

        {

            return Min == Max ? Min.ToString() : Min + "-" + Max;

        }

    }

}