QueryableHelper

using System;

using System.Collections.Generic;

using System.Linq;

using System.Linq.Expressions;

using System.Reflection;

using System.Text;

namespace Oyang.Tool

{

    public class QueryableHelper

    {

        public static IQueryable<TSource> WhereIf<TSource>(IQueryable<TSource> source, bool isTrue, Expression<Func<TSource, bool>> predicate)

        {

            if (isTrue)

            {

                source = source.Provider.CreateQuery<TSource>(

                Expression.Call(

                    null,

                    Where_TSource_2(typeof(TSource)),

                    source.Expression, Expression.Quote(predicate)

                    ));

            }

            return source;

        }

        private static MethodInfo s_Where_TSource_2;

        private static MethodInfo Where_TSource_2(Type TSource) =>

             (s_Where_TSource_2 ??

             (s_Where_TSource_2 = new Func<IQueryable<object>, Expression<Func<object, bool>>, IQueryable<object>>(Queryable.Where).GetMethodInfo().GetGenericMethodDefinition()))

              .MakeGenericMethod(TSource);

        public static List<TSource> ToPageList<TSource>(IQueryable<TSource> source, ref int pageIndex, int pageSize, string sortField, bool isAsc, out int totalCount)

        {

            totalCount = source.Count();

            int pageCount = totalCount % pageSize ==  ? totalCount / pageSize : totalCount / pageSize + ;

            if (pageCount >  && pageIndex > pageCount)

            {

                pageIndex = pageCount;

            }

            var param = Expression.Parameter(typeof(TSource));

            var body = Expression.Property(param, sortField);

            dynamic keySelector = Expression.Lambda(body, param);

            source = isAsc ? Queryable.OrderBy(source, keySelector) : Queryable.OrderByDescending(source, keySelector);

            source = source.Skip((pageIndex - ) * pageSize).Take(pageSize);

            return source.ToList();

        }

        public static List<TSource> ToPageList<TSource>(IQueryable<TSource> source, IPagination p)

        {

            int pageIndex = p.PageIndex;

            List<TSource> temp = ToPageList<TSource>(source, ref pageIndex, p.PageSize, p.SortField, p.IsAsc, out int totalCount);

            p.TotalCount = totalCount;

            p.PageIndex = pageIndex;

            return temp;

        }

    }

}