Permutation.cs

public class Permutation
{
  public static IEnumerable<string> GetPermutations(string s)
  {
      if (s.Length > 1)
          return from ch in s
                 from permutation in GetPermutations(s.Remove(s.IndexOf(ch), 1))
                 select string.Format("{0}{1}", ch, permutation);

      else
          return new string[] { s };
  }
}