.NET MVC Linq 分页与 PagedList

Question

在我的 MVC 应用程序中，我使用 LINQ 从 DB 中检索数据，并使用 PagedList 进行分页。在代码块之后，我有几个问题需要帮助。

我从缓存或数据库中检索数据的功能：

public NewsPagedListDTO GetNewsFromCacheOrDB(int pageSize, int? newsID, int? page, string newsTitle, int? categoryID, int? orderByTitle, int? orderByPublisher, int? orderByDate, int? orderByCategory)
            {
            DataCache cache = new DataCache("default");
            object cacheNews = cache.Get("cacheNews");

            List<News> news = new List<News>();


            if (cacheNews == null)
            {
                news = (from n in DB.News
                        select n).ToList();

            //Only cache if no parameters was provided
            if (newsID == null && newsTitle == null && categoryID == null && orderByTitle == null && orderByPublisher == null &&
                orderByDate == null && orderByCategory == null)
                cache.Add("cacheNews", news);
            }
            }
            else
            {
                news = (List<News>)cacheNews;
            }

            if (newsID != null)
                news = news.Where(n => n.NewsID == newsID).ToList();

            if (categoryID != null)
                news = news.Where(n => n.CategoryID == categoryID).ToList();

            if (newsTitle != null)
                news = news.Where(n => n.Title == newsTitle).ToList();

            if (orderByTitle != null)
                if (orderByTitle == 0)
                    news = news.OrderBy(n => n.Title).ToList();
                else
                    news = news.OrderByDescending(n => n.Title).ToList();

            if (orderByPublisher != null)
                if (orderByPublisher == 0)
                    news = news.OrderBy(n => n.PublishedByFullName).ToList();
                else
                    news = news.OrderByDescending(n => n.PublishedByFullName).ToList();

            if (orderByDate != null)
                if (orderByDate == 0)
                    news = news.OrderByDescending(n => n.DatePublished).ToList();
                else
                    news = news.OrderBy(n => n.DatePublished).ToList();

            if (orderByCategory != null)
                if (orderByCategory == 0)
                    news = news.OrderBy(n => n.CategoryToString).ToList();
                else
                    news = news.OrderByDescending(n => n.CategoryToString).ToList();


            List<NewsDTO> newsDTO = new List<NewsDTO>();

            foreach (var item in news)
            {
                NewsDTO newsDTOtemp = new NewsDTO();

                newsDTOtemp.BlobName = item.BlobName;
                newsDTOtemp.DatePublished = item.DatePublished;
                newsDTOtemp.NewsID = item.NewsID;
                newsDTOtemp.PreviewText = item.PreviewText;
                newsDTOtemp.PublishedByEmail = item.PublishedByEmail;
                newsDTOtemp.PublishedByFullName = item.PublishedByFullName;
                newsDTOtemp.PublishedByID = item.PublishedByID;
                newsDTOtemp.Title = item.Title;
                newsDTOtemp.CategoryID = item.Category.CategoryID;
                newsDTOtemp.CategoryToString = item.Category.Name;

                newsDTO.Add(newsDTOtemp);
            }

            //Pagination
            NewsPagedListDTO newsResultDTO = new NewsPagedListDTO();
            newsResultDTO.NewsDTO = (PagedList<NewsDTO>)newsDTO.ToPagedList(page ?? 1, pageSize);

            return newsResultDTO;
        }

我认为的分页：

@Html.PagedListPager(Model.NewsPagedListDTO.NewsDTO, page => Url.Action("News", new
   {
       page,
       newsTitle = Request.QueryString["NewsTitle"],
       categoryID = Request.QueryString["categoryID"],
       orderByTitle = Request.QueryString["orderByTitle"],
       orderByPublisher = Request.QueryString["orderByPublisher"],
       orderByDate = Request.QueryString["orderByDate"],
       orderByCategory = Request.QueryString["orderByCategory"]
   }),
    new PagedListRenderOptions()
    {
        Display = PagedListDisplayMode.IfNeeded,
        MaximumPageNumbersToDisplay = 5,
        DisplayEllipsesWhenNotShowingAllPageNumbers = false,
        DisplayLinkToPreviousPage = PagedListDisplayMode.Never,
        DisplayLinkToNextPage = PagedListDisplayMode.Never,
        LinkToFirstPageFormat = String.Format("«"),
        LinkToLastPageFormat = String.Format("»")
    })

问题

1. 这是我第一次使用 PagedList。在检索到完整结果时进行回发以更改页面有什么意义？那么客户端分页不是更好吗？目前我正在从数据库中检索所有帖子：

   news = (来自 DB.News 中的 n 选择 n).ToList();

   并在检索到数据后，使用参数进行排序..

   当然结果很容易缓存，但是..我宁愿只获取一页的数据。

2. 如何仅使用可选参数获取当前页面的数据？我以前使用过存储过程，但我认为 PagedList 不可能。

3. 如何在我的 LINQ 查询中为可选参数提供更简洁的代码？我不喜欢所有那些 if 语句..

Answer 1

问题是你必须Skip items 然后Take(pageSize)

var pagedNews = DB.News.Skip((currentPage - 1) * pageSize).Take(pageSize).ToList();

假设您有 5 个项目/页。

如果您在第 1 页

(1 - 1) * 5 = 0 所以跳过零个项目并取 5 个

如果您在第 2 页

(2 - 1) * 5 = 5 所以跳过 5 个项目并拿 5 个

您的参数是Nullable，因此您可能必须在参数上设置一个默认条件，例如如果为NULL，则为PageSize = 5 和PageNumber = 1

int pageSize, int? newsID, int? page

编辑：

代替：

if (cacheNews == null)
{
                news = (from n in DB.News
                        select n).ToList();

...........
}

使用这个：

// 在进行分页之前，您必须使用 OrderBy()：

// Read as Queryable()

var pagedNews = DB.News.AsQueryable();

// Apply OrderBy Logic
pagedNews = pagedNews.OrderBy(); 

//ApplyPagination
pagedNews = pagedNews.Skip((currentPage - 1) * pageSize).Take(pageSize).ToList();

订购方式

您不需要将 OrderBy 列作为单独的字符串传递。

传递一个字符串，例如selectedSortBy来自视图，

我已经创建了一个 Helper 方法：

using System;
using System.Linq;
using System.Linq.Expressions;

namespace Common.Helpers
{
    public static class PaginationHelper
    {
        public static IQueryable<T> ApplyPagination<T>(IQueryable<T> source, Pagination pagination)
        {
            var sortDirection = pagination.SortDirection == SortDirectionEnum.Ascending ? "OrderBy" : "OrderByDescending";
            var orderBy = pagination.SortBy ?? pagination.DefaultSortBy;

            return source.OrderBy(orderBy, sortDirection).Skip((pagination.PageNumber - 1) * pagination.PageSize).Take(pagination.PageSize);
        }

        public static IQueryable<T> OrderBy<T>(this IQueryable<T> source, string ordering, string sortDirection, params object[] values)
        {
            var type = typeof(T);
            var property = type.GetProperty(ordering);
            var parameter = Expression.Parameter(type, "p");
            var propertyAccess = Expression.MakeMemberAccess(parameter, property);
            var orderByExp = Expression.Lambda(propertyAccess, parameter);
            var resultExp = Expression.Call(typeof(Queryable), sortDirection, new Type[] { type, property.PropertyType }, source.Expression, Expression.Quote(orderByExp));
            return source.Provider.CreateQuery<T>(resultExp);
        }
    }
}

分页模型+枚举：

namespace Common.Helpers
{
    public class Pagination
    {
        public SortDirectionEnum SortDirection { get; set; }
        public string SortBy { get; set; }
        public int TotalRecords { get; set; }
        public int NumberOfPages { get; set; }
        public int PageSize { get; set; }
        public int PageNumber { get; set; }

        public string DefaultSortBy { get; set; }

        public string ReloadUrl { get; set; }
        public string TargetDiv { get; set; }

        public Pagination()
        {

        }

        public Pagination(string reloadUrl, string targetDiv, int totalRecords, int numberOfPages)
        {
            ReloadUrl = reloadUrl;
            TargetDiv = targetDiv;
            PageSize = 10;
            PageNumber = 1;
        }
    }

    public enum SortDirectionEnum
    {
        Ascending = 1,
        Descending = 2
    }
}

然后像这样调用您的查询：

var items = DB.News.AsQueryable();

items = PaginationHelper.ApplyPagination(items, PAGINATION_MODEL);