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，我想要一些帮助。
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。在检索完整结果时，更改页面的回发是什么意义？那么客户端分页不是更好吗？目前我正在从DB中检索所有帖子：
news =（来自DB.News中的n 选择n）.ToList（）;
检索数据后，使用参数进行排序..
当然结果很容易缓存但是......我只是只获取一页的数据。
我如何只使用可选参数获取当前页面的数据？我之前已经使用过存储过程，但我不认为它可以使用PagedList。
如何在LINQ查询中为可选参数提供更清晰的代码？我不喜欢所有那些陈述......
答案 0 :(得分：1)
问题是你必须Skip项，然后是Take(pageSize)
var pagedNews = DB.News.Skip((currentPage - 1) * pageSize).Take(pageSize).ToList();
所以假设你有5件/页。
如果你在第1页
(1 - 1) * 5 = 0所以跳过零项并取5
如果你在第2页
(2 - 1) * 5 = 5所以跳过5个项目并取5个
您的参数为Nullable，因此您可能必须在参数上设置默认条件，如果为NULL PageSize = 5和PageNumber = 1
int pageSize, int? newsID, int? page
修改
而不是：
if (cacheNews == null)
{
news = (from n in DB.News
select n).ToList();
...........
}
使用此：
//在进行分页之前你必须使用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();
ORDER BY
您不需要将OrderBy列作为单独的字符串传递。
传递一个字符串，例如来自View的selectedSortBy，
我创建了一个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);