using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        List<string> numbers = new List<string>(){"1","2","3","4","5"};

        List<string> possibleCombination = GetCombination(numbers, new List<string>(), "");

        Console.Write(string.Join(", ", possibleCombination.Distinct().OrderBy(itm => itm)));

    }

    static List<string> GetCombination(List<string> list, List<string> combinations, string sumNum, bool addNumberToResult = false)

    {

        if (list.Count == 0) {

            return combinations;

        }

        string tmp;

        for (int i = 0; i <= list.Count - 1; i++) {

            tmp = string.Concat(sumNum , list[i]);

            if(addNumberToResult){

                combinations.Add(tmp);

            }

            List<string> tmp_list = new List<string>(list);

            tmp_list.RemoveAt(i);

            GetCombination(tmp_list,combinations,tmp, true);

        }

        return combinations;

    }

}