Eric Lippert's challenge “comma-quibbling”, best answer?

Question

I wanted to bring this challenge to the attention of the stackoverflow community. The original problem and answers are here. BTW, if you did not follow it before, you should

Answer 1

Disclaimer: I used this as an excuse to play around with new technologies, so my solutions don't really live up to the Eric's original demands for clarity and maintainability.

Naive Enumerator Solution (I concede that the foreach variant of this is superior, as it doesn't require manually messing about with the enumerator.)

public static string NaiveConcatenate(IEnumerable sequence)
{
    StringBuilder sb = new StringBuilder();
    sb.Append('{');

    IEnumerator enumerator = sequence.GetEnumerator();

    if (enumerator.MoveNext())
    {
        string a = enumerator.Current;
        if (!enumerator.MoveNext())
        {
            sb.Append(a);
        }
        else
        {
            string b = enumerator.Current;
            while (enumerator.MoveNext())
            {
                sb.Append(a);
                sb.Append(", ");
                a = b;
                b = enumerator.Current;
            }
            sb.AppendFormat("{0} and {1}", a, b);
        }
    }

    sb.Append('}');
    return sb.ToString();
}

Solution using LINQ

public static string ConcatenateWithLinq(IEnumerable sequence)
{
    return (from item in sequence select item)
        .Aggregate(
        new {sb = new StringBuilder("{"), a = (string) null, b = (string) null},
        (s, x) =>
            {
                if (s.a != null)
                {
                    s.sb.Append(s.a);
                    s.sb.Append(", ");
                }
                return new {s.sb, a = s.b, b = x};
            },
        (s) =>
            {
                if (s.b != null)
                    if (s.a != null)
                        s.sb.AppendFormat("{0} and {1}", s.a, s.b);
                    else
                        s.sb.Append(s.b);
                s.sb.Append("}");
                return s.sb.ToString();
            });
}

Solution with TPL

This solution uses a producer-consumer queue to feed the input sequence to the processor, whilst keeping at least two elements buffered in the queue. Once the producer has reached the end of the input sequence, the last two elements can be processed with special treatment.

In hindsight there is no reason to have the consumer operate asynchronously, which would eliminate the need for a concurrent queue, but as I said previously, I was just using this as an excuse to play around with new technologies :-)

public static string ConcatenateWithTpl(IEnumerable sequence)
{
    var queue = new ConcurrentQueue();
    bool stop = false;

    var consumer = Future.Create(
        () =>
            {
                var sb = new StringBuilder("{");
                while (!stop || queue.Count > 2)
                {
                    string s;
                    if (queue.Count > 2 && queue.TryDequeue(out s))
                        sb.AppendFormat("{0}, ", s);
                }
                return sb;
            });

    // Producer
    foreach (var item in sequence)
        queue.Enqueue(item);

    stop = true;
    StringBuilder result = consumer.Value;

    string a;
    string b;

    if (queue.TryDequeue(out a))
        if (queue.TryDequeue(out b))
            result.AppendFormat("{0} and {1}", a, b);
        else
            result.Append(a);

    result.Append("}");
    return result.ToString();
}

Unit tests elided for brevity.