public static class QueryableExtensions

    {

        #region 内存分页

        /// <summary>

        /// 返回对象分页列表

        /// </summary>

        /// <typeparam name="T">对象类型</typeparam>

        /// <param name="dataSource">已排序的数据源</param>

        /// <param name="pageIndex">页码，1开始</param>

        /// <param name="pageSize">页条数</param>

        /// <returns>对象分页列表</returns>

        public static IPagedList<T> ToPagedList<T>(this IList<T> dataSource, int pageIndex, int pageSize)

            where T : class

        {

            pageIndex = pageIndex <=  ?  : pageIndex;

            var pagedList = new PagedList<T>

            {

                PageIndex = pageIndex,

                PageSize = pageSize,

                TotalCount = dataSource.Count,

                Entities = dataSource.Skip((pageIndex - ) * pageSize).Take(pageSize).ToList()

            };

            return pagedList;

        }

        /// <summary>

        /// 返回对象分页列表

        /// </summary>

        /// <typeparam name="T">对象类型</typeparam>

        /// <param name="dataSource">已排序的数据源</param>

        /// <param name="pager">分页器对象；当为空时，分页器取默认值</param>

        /// <returns>对象分页列表</returns>

        public static IPagedList<T> ToPagedList<T>(this IList<T> dataSource, Pager pager) where T : class

        {

            pager = pager ?? new Pager();

            return dataSource.ToPagedList(pager.PageIndex, pager.PageSize);

        }

        #endregion

        #region IQueryable分页

        /// <summary>

        /// 返回对象分页列表

        /// </summary>

        /// <typeparam name="T">对象类型</typeparam>

        /// <param name="dataSource">已排序的数据源</param>

        /// <param name="pageIndex">页码，1开始</param>

        /// <param name="pageSize">页条数</param>

        /// <returns>对象分页列表</returns>

        public static IPagedList<T> ToPagedList<T>(this IQueryable<T> dataSource, int pageIndex, int pageSize)

            where T : class

        {

            pageIndex = pageIndex <=  ?  : pageIndex;

            var pagedList = new PagedList<T>

            {

                PageIndex = pageIndex,

                PageSize = pageSize,

                TotalCount = dataSource.Count(),

                Entities = dataSource.Skip((pageIndex - ) * pageSize).Take(pageSize).ToList()

            };

            return pagedList;

        }

        /// <summary>

        /// 返回对象分页列表

        /// </summary>

        /// <typeparam name="T">对象类型</typeparam>

        /// <param name="dataSource">已排序的数据源</param>

        /// <param name="pager">分页器对象；当为空时，分页器取默认值</param>

        /// <returns>对象分页列表</returns>

        public static IPagedList<T> ToPagedList<T>(this IQueryable<T> dataSource, Pager pager) where T : class

        {

            pager = pager ?? new Pager();

            return dataSource.ToPagedList(pager.PageIndex, pager.PageSize);

        }

        #endregion

        /// <summary>

        /// 当满足特定条件时执行查询

        /// </summary>

        /// <typeparam name="T">对象类型</typeparam>

        /// <param name="source">查询对象</param>

        /// <param name="condition">需要满足的条件</param>

        /// <param name="predicate">满足条件时执行的查询表达式</param>

        /// <returns>结果集</returns>

        public static IQueryable<T> WhereIf<T>(this IQueryable<T> source, bool condition,

            Expression<Func<T, bool>> predicate)

        {

            return condition ? source.Where(predicate) : source;

        }

    }

 /// <summary>

    /// 分页器

    /// </summary>

    public class Pager

    {

        /// <summary>

        /// 分页索引（默认为1）

        /// </summary>

        public int PageIndex { get; set; } = ;

        /// <summary>

        /// 分页大小（默认为10）

        /// </summary>

        public int PageSize { get; set; } = ;

    }