Permutations of a String

using System;

namespace permutation
{
    class Program
    {
        static void Main(string[] args)
        {
            string derp = "1234";
            perm(derp, 0, derp.Length);

            Console.ReadKey();
        }

        static void perm(string a, int k, int n)
        {
            if (k == n)
            {
                Console.WriteLine(a);
            }
            else
            {
                // Strings are immutable objects, so you can't replace a given character in the string.
                char[] charArr = a.ToCharArray();
                for (int i = k; i < n; i++)
                {
                    var t = charArr[k];
                    charArr[k] = charArr[i];
                    charArr[i] = t;
                    perm(new string(charArr), k + 1, n);
                }
            }
        }
    }
}