C# Class is IEnumerable AND an IEnumerator at the same time. What are the issues with this?

Question

I have a class called GenericPermutations that is both enumerable and an enumerator. Its job is to take an ordered list of objects and iterate through each permutation of t

Answer 1

Reduce your confusion (?) by using the generic versions of IEnumerable and IEnumerator.

A permutation enumerable is IEnumerable>. So you might have something like

IEnumerable> GetPermutations(IEnumerable sequence)
{
    return new Permuter(sequence);
}

and

public class Permuter : IEnumerable> { ... }

Furthermore, I've seen more than one case where a single type implemented both IEnumerable and IEnumerator; its GetEnumerator method was simply return this;.

I think such a type would need to be a struct, though, because if it were a class you'd have all sorts of problems if you called GetEnumerator() a second time before the first enumeration was completed.

EDIT: Consuming the permuter

var permuter = GetPermutations(sequence);
foreach (var permutation in permuter)
{
    foreach (var item in permutation)
        Console.Write(item + "; ");
    Console.WriteLine();
}

Assuming the input sequence is { 1, 2, 3 }, the output is

1; 2; 3; 
1; 3; 2; 
2; 1; 3; 
2; 3; 1; 
3; 1; 2; 
3; 2; 1; 

EDIT:

Here's a super-inefficient implementation to illustrate the suggestion:

public class Permuter : IEnumerable>
{
    private readonly IEnumerable _sequence;

    public Permuter(IEnumerable sequence)
    {
        _sequence = sequence;
    }

    public IEnumerator> GetEnumerator()
    {
        foreach(var item in _sequence)
        {
            var remaining = _sequence.Except(Enumerable.Repeat(item, 1));
            foreach (var permutation in new Permuter(remaining))
                yield return Enumerable.Repeat(item, 1).Concat(permutation);
        }
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }
}