C# Permutation Generator

cspermgen.cs

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

namespace CSharpPermutations
{

    public interface IPermutable<T>
    {
        ISet<T> GetRange();
    }

    public class Digits : IPermutable<int>
    {
        public ISet<int> GetRange()
        {
            ISet<int> set = new HashSet<int>();
            for (int i = 0; i < 10; ++i)
                set.Add(i);
            return set;
        }
    }

    public class AlphaNumeric : IPermutable<char>
    {
        public ISet<char> GetRange()
        {
            ISet<char> set = new HashSet<char>();
            set.Add('0');
            set.Add('1');
            set.Add('2');
            set.Add('3');
            set.Add('4');
            set.Add('5');
            set.Add('6');
            set.Add('7');
            set.Add('8');
            set.Add('9');
            set.Add('a');
            set.Add('b');

            return set;
        }
    }


    public class PermutationGenerator<T,P> : IEnumerable<string>
        where P : IPermutable<T>, new()
    {

        public PermutationGenerator(int number)
        {
            this.number = number;
            this.range = new P().GetRange();
        }

        public IEnumerator<string> GetEnumerator()
        {
            foreach (var item in Permutations(0,0))
            {
                yield return item.ToString();
            }
        }

        IEnumerator IEnumerable.GetEnumerator()
        {
            foreach (var item in Permutations(0,0))
            {
                yield return item;
            }
        }

        private IEnumerable<StringBuilder> Permutations(int n, int k)
        {
            if (n == number)
                yield return new StringBuilder();

            foreach (var element in range.Skip(k))
            {
                foreach (var result in Permutations(n + 1, k + 1))
                {
                    yield return new StringBuilder().Append(element).Append(result);
                }
            }
        }

        private int number;
        private ISet<T> range;

    }

    class MainClass
    {
        public static void Main(string[] args)
        {
            foreach (var element in new PermutationGenerator<char, AlphaNumeric>(2))
            {
                Console.WriteLine(element);
            }
        }
    }
}