Permutations.cs

// Generates partial permutations for a given string:
private HashSet<string> GeneratePartialPermutation(string word)
{
    return new HashSet<string>(Enumerable.Range(0, word.Length)
        .Select(i => word.Remove(i, 1).Insert(0, word[i].ToString())));
}

// Entry point for the LINQ recipe:
void Main()
{
    // Stores all the permutations computed in HashSet data structure.
    // It also generates the first partial permutations:
    HashSet<string> perms = GeneratePartialPermutation("abcd");

    // Generates permutations by traversing all possible permutations of
    // the given string -"abcd"-:
    Enumerable.Range(0, 2)
        .ToList()
        .ForEach
        (
            c =>
            {
                // Permutations in forward direction:
                Enumerable.Range(0, perms.Count())
                .ToList()
                .ForEach
                (
                    i => GeneratePartialPermutation(
                        perms.ElementAt(i))
                        .ToList()
                        .ForEach(p => perms.Add(p))
                );

                // Permutations in reverse direction:
                Enumerable.Range(0, perms.Count())
                .ToList()
                .ForEach
                (
                    i => GeneratePartialPermutation(new String
                        (perms.ElementAt(i).ToCharArray()
                            .Reverse().ToArray())
                )
                .ToList().ForEach(p => perms.Add(p)));
            }
        );

    // Displays all permutations:
    perms.OrderBy(p => p).Dump("Permutations of 'abcd'");
}