c# - LINQ 如何返回最后日期以及第一次和最后一次计数之间的差异

Question

我有一个包含以下列和示例数据的表:

acteename   updated_at              count (count is not sorted in db)

dev-52      2/7/2020 5:56:43 PM     1
dev-52      2/7/2020 5:56:52 PM     2
dev-52      2/7/2020 5:57:00 PM     3
dev-52      2/7/2020 5:57:49 PM     4
dev-52      2/7/2020 5:59:19 PM     5
dev-52      2/7/2020 6:01:51 PM     6
dev-52      2/7/2020 6:06:21 PM     7
dev-52      2/7/2020 6:14:51 PM     8
dev-52      2/7/2020 6:31:23 PM     9
dev-52      2/7/2020 6:47:54 PM     10
dev-52      2/7/2020 7:04:26 PM     11
dev-52      2/7/2020 7:20:58 PM     12

我想返回 first 之间的差异和 last按 updated_at 排序的计数最后 updated_at where acteename等于我从不同表中获得的某个名称

IQueryable<Events> list = _DBcontext.Events.GroupBy(p => p.ActeeName)
                            .Select(g => new
                            {
                                acteename = g.Key,
                                lastupdated = g.OrderByDescending(p => p.UpdatedAt).First(),
                                firstcount = g.OrderBy(p => p.UpdatedAt).First().Count,
                                lastcount = g.OrderByDescending(p => p.UpdatedAt).First().Count
                            })
                            .Select(x => new { x.acteename,x.lastupdated, diff = x.lastcount - x.firstcount })
                            .Where(a => a.acteename == item.AppName)
                            .ToList();

得到以下错误:

Cannot implicitly convert type 'System.Collections.Generic.List<<anonymous type: string acteename,
TestApp.Models.Events lastupdated, int diff>>' to 'System.Linq.IQueryable<TestApp.Models.Events>'. An explicit conversion exists (are you missing a cast?)

预期输出:

dev-52      2/7/2020 7:20:58 PM     11

有点卡在这不确定 LINQ 查询是否正确，任何帮助都会有用:)

更新:

我运行的确切查询:

var list = from p in _DBcontext.AppProcessCrashEvents
                                   where p.Actee == item.AppGuid
                                   group p by p.Actee
            into gList
                                   select new
                                   {
                                       acteenamex = gList.Key,
                                       lastUpdated_at = gList.OrderBy(x => x.UpdatedAt).Last().UpdatedAt,
                                       diff = gList.OrderBy(x => x.UpdatedAt).Last().CrashCount -
                                                             gList.OrderBy(x => x.UpdatedAt).First().CrashCount
                                   };

Answer 1

人们可以在一开始就应用过滤器本身。使用 group by与 into然后最后选择你需要的。由于它创建了一个匿名对象，因此最好以“var”开头。

已编辑:获取内存中的数据然后应用 group by避免在与数据库端分组映射时出现任何问题。

var list =  (from p in _DBcontext.Events
            where p.acteename == item.AppName select p).ToList();

var fList = (from p in list 
            group p by p.acteename
            into gList
            select new
            {
               acteenamex = gList.Key,
               lastUpdated_at = gList.OrderBy(x => x.updated_at).Last().updated_at,
               diff = gList.OrderBy(x => x.updated_at).Last().count - 
                                      gList.OrderBy(x => x.updated_at).First().count
            }).ToList();