Convert list to number range string
1,3,5,6,7,8,9,10,12:

1,3,5-10,12

static string numListToRangeStr(List<int> numList)
{
    StringBuilder retString = new StringBuilder();
    numList.Sort();

    bool inRangeFind = false;
    int firstInRange = numList[0];
    int lastNumber = firstInRange;
    bool first = true;

    for (int i = 1; i < numList.Count; i++)
    {
        if (numList[i] == (lastNumber + 1))
        {
            inRangeFind = true;
        }
        else
        {
            if (inRangeFind)
            {
                if (!first)
                {
                    retString.Append(",");
                }
                retString.Append(firstInRange);
                retString.Append("-");
            }
            else
            {
               if (!first)
                {
                    retString.Append(",");
                }
            }

            retString.Append(lastNumber);

            firstInRange = numList[i];
            inRangeFind = false;
            first = false;
        }

        lastNumber = numList[i];
    }


    if (inRangeFind)
    {
        if (!first)
        {
            retString.Append(",");
        }
        retString.Append(firstInRange);
        retString.Append("-");
    }
    else
    {
        if (!first)
        {
            retString.Append(",");
        }
    }
    retString.Append(lastNumber);

    return retString.ToString();
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication37 {
  public static class Program {
    static void Main(string[] args) {
      var numList=new[] {1, 3, 5, 6, 7, 8, 9, 10, 12};
      Console.WriteLine(numListToPossiblyDegenerateRanges(numList).Select(r => PrettyRange(r)).Intersperse(","));
    }

    /// <summary>
    /// e.g. 1,3,5,6,7,8,9,10,12
    /// becomes
    /// (1,1),(3,3),(5,10),(12,12)
    /// </summary>
    public static IEnumerable<Tuple<int,int>> numListToPossiblyDegenerateRanges(IEnumerable<int> numList) {
      Tuple<int, int> currentRange=null;
      foreach(var num in numList) {
        if(currentRange==null) {
          currentRange=Tuple.Create(num, num);
        } else if(currentRange.Item2==num-1) {
          currentRange=Tuple.Create(currentRange.Item1, num);
        } else {
          yield return currentRange;
          currentRange=Tuple.Create(num, num);
        }
      }
      if(currentRange!=null) {
        yield return currentRange;
      }
    }

    /// <summary>
    /// e.g. (1,1) becomes "1"
    /// (1,3) becomes "1-3"
    /// </summary>
    /// <param name="range"></param>
    /// <returns></returns>
    public static string PrettyRange(Tuple<int,int> range) {
      if(range.Item1==range.Item2) {
        return range.Item1.ToString();
      }
      return string.Format("{0}-{1}", range.Item1, range.Item2);
    }

    public static string Intersperse(this IEnumerable<string> items, string interspersand) {
      var currentInterspersand="";
      var result=new StringBuilder();
      foreach(var item in items) {
        result.Append(currentInterspersand);
        result.Append(item);
        currentInterspersand=interspersand;
      }
      return result.ToString();
    }
  }
}

string s = "1,2,3,4,5,7,8,9,10,11,12,13";
        string[] ints = s.Split(',');
        StringBuilder result = new StringBuilder();

        int num, last = -1;
        bool dash = false;

        for (int ii = 0; ii < ints.Length; ii++)
        {
            num = Int32.Parse(ints[ii]);

            if (num - last > 1)
            {
                if (dash)
                {
                    result.Append(last);
                    dash = false;
                }
                if (result.Length > 0)
                {
                    result.Append(",");
                }
                result.Append(num);
            }
            else
            {
                if (dash == false)
                {
                    result.Append("-");
                    dash = true;
                }
            }

            last = num;

            if (dash && ii == ints.Length - 1)
            {
                result.Append(num);
            }
        }

        Console.WriteLine(result);