testlastcommented = doc.Comments.Count > 0 ? doc.Comments.Select(c => c.DateEntered).Max() : null

testlastcommented = doc.Comments.OrderByDescending(c => c.DateEntered).FirstOrDefault()

testlastcommented = doc.Comments.Select(c => c.DateEntered)
                                .OrderByDescending(x => x)
                                .FirstOrDefault()