Do sort and pager for both IQueryable<T> and IEnumerable<T> meanwhile.

PagerProduct.cs

   public class Foo 
   {
        public virtual IEnumerable<Product> Page(IEnumerable<Product> list, Model.Pager pager)
        {
            IEnumerable<Product> b;
            if (pager == null)
            {
                throw new ArgumentNullException("pager");
            }
            var q = list as IQueryable<Product>;

            // 排序
            if (pager.SortRule == Model.SortRule.ProductDefault)
            {
                if (q != null)
                {
                    q = q.OrderByDescending(p => p.Weight)
                                    .ThenBy(p => p.EffectDate)
                                    .ThenBy(p => p.TripDays);
                }
                else
                {
                    list = list.OrderByDescending(p => p.Weight)
                                    .ThenBy(p => p.EffectDate)
                                    .ThenBy(p => p.TripDays);
                }
            }
            else
            {
                if (pager.SortRule == Model.SortRule.ProductTripDays)
                {
                    if (q != null)
                    {
                        q = q.OrderBy(p => p.TripDays);
                    }
                    else
                    {
                        list.OrderBy(p => p.TripDays);
                    }
                }
                else if (pager.SortRule == Model.SortRule.ProductMinDepartureDate)
                {
                    if (q != null)
                    {
                        q = q.OrderBy(p => p.EffectDate);
                    }
                    else
                    {
                        list.OrderBy(p => p.EffectDate);
                    }
                }
                else if (pager.SortRule == Model.SortRule.ProductMinPrice)
                {
                    if (q != null)
                    {
                        q = q.OrderBy(p => p.Product_Price.Min(pr => pr.Price));
                    }
                    else
                    {
                        list.OrderBy(p => p.Product_Price.Min(pr => pr.Price));
                    }
                }
                else
                {
                    throw new ArgumentOutOfRangeException("pager.SortRule");
                }

                if (pager.Descending)
                {
                    if (q != null)
                    {
                        q = q.Reverse();
                    }
                    else
                    {
                        list = list.Reverse();
                    }
                }
            }

            int pageCount;
            if (q != null)
            {
                b = PagerHelper.TakePage(q, pager.PageSize, pager.PageIndex, out pageCount);
            }
            else
            {
                b = PagerHelper.TakePage(list, pager.PageSize, pager.PageIndex, out pageCount);
            }
            pager.PageCount = pageCount;

            return b;
        }
    }

    public class PagerHelper
    {
        public static IEnumerable<T> TakePage<T>(IEnumerable<T> list, int pageSize, int? pageIndex, out int pageCount) where T : class
        {
            int count = list.Count();
            pageCount = (int)Math.Ceiling((decimal)count / pageSize);

            int i = 0;
            if (pageIndex.HasValue)
            {
                i = pageIndex.Value;

                // fix range
                if (i < 0) i = 0;
            }

            // fix range
            if (pageIndex > pageCount)
                pageIndex = pageCount;

            int skip = i * pageSize;
            int take = pageSize;

            var q = list as IQueryable<T>;
            if (q != null)
            {
                return q.Skip(skip).Take(take);
            }
            else
            {
                return list.Skip(skip).Take(take);
            }
        }
    }