gistfile1.cs

// yield 版
IEnumerable<List<T>> Permutations<T>(IEnumerable<T> seq, int count)
{
  switch (count) {
  case 0: // special
    yield break;
  case 1: // terminate
    foreach (var t in seq)
      yield return t;
    yield break;
  default: // permutation
    var list = seq.ToList(); // defensive copy
    foreach (var head in list)
      foreach (var tail in Permutations(list.Skip(1), count - 1))
        yield return new List<T>{ head }.Concat(tail).ToList();
    yield break;
  }
}

// LINQ版
IEnumerable<List<T>> Permutations<T>(IEnumerable<T> seq, int count)
{
  switch (count) {
  case 0: // special
    return Enumerable.Empty<List<T>>();
  case 1: // terminate
    return from t in seq
      select new List<T>{ t };
  default: // permutation
    var list = seq.ToList(); // defensive copy
    return from head in list
      from tail in Permutations(list.Skip(1), count - 1)
        select new List<T>{ head }.Concat(tail).ToList();
  }
}