使用 PagedList 的 .NET MVC Linq 分页

Question

在我的 MVC 应用程序中，我使用 LINQ 从数据库中检索数据并使用 PagedList 进行分页。在代码块之后，我有几个问题需要一些帮助。

我从缓存或数据库中检索数据的函数：

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

这是我第一次使用 PagedList。当检索到完整结果时，通过回传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：

新闻=（来自DB.News中的n select n).ToList();

获取数据后，根据参数排序..

当然结果很容易缓存但是..我宁愿只获取一页的数据。
如何使用可选参数只获取当前页面的数据？我以前为此使用过存储过程，但我认为 PagedList 不可能。
如何让我的 LINQ 查询中的可选参数代码更清晰？我不喜欢所有这些 if 语句..

Answer 1

问题是你必须 Skip 项然后 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 所以跳过零项并取 5

如果您在第 2 页

(2 - 1) * 5 = 5 所以跳过 5 项并取 5

你的参数是 Nullable 所以你可能必须在你的参数上设置一个默认条件，比如如果 NULL 那么 PageSize = 5 和 PageNumber = 1

int pageSize, int? newsID, int? page

编辑：

而不是：

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

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

使用这个：

// You will have to OrderBy() before doing the pagination:

// 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，

我创建了一个辅助方法：

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);

使用 PagedList 的 .NET MVC Linq 分页

.NET MVC Linq pagination with PagedList

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