using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        List<string> list1 =  new List<string>()

        {

            "1","2","3","4"

        };

        //list1.Add(null);

        list1.Add("5");

        string  finalStr = "";

        //list1.RemoveAll(string.IsNullOrWhiteSpace);

        //string str = string.Join(",", list1);

        foreach(var str1 in list1)

        {

            finalStr = finalStr + str1 + "-";

            foreach(var str2 in list1)

            {

                finalStr = finalStr + str2;             

            }

            finalStr = finalStr +  "#";

        }

        Console.WriteLine("Hello World:"+ finalStr);

    }

}