using System;

using System.Collections.Generic;

using System.Linq;          

public class Program

{

    public static void Main()

    {

        List<string> lstSource= new List<string>(){

            "1","16","2","3","5","18","6","15","7","8","10","10","11","15","17"

        };

        var listNotDuplicate = lstSource.Distinct().ToList();

        var rest= ConvertToString(listNotDuplicate);

        Console.Write(rest);

    }

     public static string ConvertToString(List<string> input)

        {

            //Result list

            List<string> ret = new List<string>();

            //Check input list

            if (input.Count == 0) return "";

            // Convert to Int32 array and sort it to ascending

            int[] inputInt = input.Select(x => Int32.Parse(x)).OrderBy(x => x).ToArray();

            //Start and End of each result element

            var start = inputInt[0];

            var end = start;

            //Variable to check if must be add to result, initialize it to false

            var add =false;

            for (int i = 1; i < inputInt.Length; i++)

            {

                // Current element

                var current = inputInt[i];

                // If current element = end + 1.

                if (current == end + 1)

                {

                    end = current;

                }

                else

                {

                    add=true;

                }

                if(add==true || i== inputInt.Length-1)

                {

                    if (start != end)

                    {

                        if(start + 1!= end){

                            ret.Add(start + ".." + end);

                        }

                        else{

                            ret.Add(start+"");

                            ret.Add(end+"");

                        }

                    }

                    else

                    {

                        ret.Add(start + "");

                    }

                    start = current;

                    end = current;

                    add=false;

                }

            }

            return string.Join(";",ret);

        }

​

}