String sorting program in c#.net

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

namespace ConsoleApplication1

{

    class Program

    {

        static void Main(string[] args)

        {

            Console.WriteLine("Enter a string");

            StringBuilder s1 = new StringBuilder(Console.ReadLine());

            char a;

            int i, j;

            for (i = 0; i < s1.Length; i++)

            {

                for (j = i; j < s1.Length; j++)

                {

                    if (s1[i] > s1[j])

                    {

                        a = s1[i];

                        s1[i] = s1[j];

                        s1[j] = a;

                    }

                }

                Console.Write(s1[i]);

            }

            Console.ReadLine();

        }

    }

}