gistfile1.cs

Func<int[], string> toRanges = source => source.Length == 0 ? "" :
    String.Join(",", source.Skip(1).Aggregate(Enumerable.Repeat(Tuple.Create(source[0], source[0]), 1),
        (combined, i) => combined.Last().Item2 == i - 1 ?
            combined.Take(combined.Count() - 1).Concat(new[] { Tuple.Create(combined.Last().Item1, i) }) :
            combined.Concat(new[] { Tuple.Create(i, i) })
    ).Select(range => range.Item1 == range.Item2 ? range.Item1.ToString() : range.Item1 + "-" + range.Item2));