C# LINQ相当复杂的排序

Question

C# LINQ相当复杂的排序

3

我有一个对象列表，需要将其转换为相当复杂的排序顺序。

尽管我在其他编程语言方面并不是新手，但对于linq和c#/.net我完全是个新手，因此我希望能得到一些方向性的提示。

我的列表（简化版）如下：

List[
    {nr:  1, originatesFrom: null, lastChanged: DateTime(2018,5,3)},
    {nr:  2, originatesFrom: null, lastChanged: DateTime(2018,5,3)},
    {nr:  3, originatesFrom: null, lastChanged: DateTime(2018,5,3)},
    {nr:  4, originatesFrom: 1,    lastChanged: DateTime(2018,5,1)},
    {nr:  5, originatesFrom: 2,    lastChanged: DateTime(2018,5,1)},
    {nr:  6, originatesFrom: 1,    lastChanged: DateTime(2018,5,7)},
    {nr:  7, originatesFrom: 1,    lastChanged: DateTime(2018,5,4)},
    {nr:  8, originatesFrom: 3,    lastChanged: DateTime(2018,5,13)},
    {nr:  9, originatesFrom: 1,    lastChanged: DateTime(2018,5,13)},
    {nr: 10, originatesFrom: 3,    lastChanged: DateTime(2018,5,10)},
    {nr: 11, originatesFrom: 3,    lastChanged: DateTime(2018,5,18)}
]

我需要将其转化为类似于以下的内容：
我需要将其转化为可读性更强的文本。

List[
    {nr:  5, originatesFrom: 2,    lastChanged: DateTime(2018,5,1)},
    {nr:  2, originatesFrom: null, lastChanged: DateTime(2018,5,3)},
    {nr:  9, originatesFrom: 1,    lastChanged: DateTime(2018,5,13)},
    {nr:  6, originatesFrom: 1     lastChanged: DateTime(2018,5,7)},
    {nr:  7, originatesFrom: 1,    lastChanged: DateTime(2018,5,4)},
    {nr:  4, originatesFrom: 1,    lastChanged: DateTime(2018,5,1)},
    {nr:  1, originatesFrom: null, lastChanged: DateTime(2018,5,3)},
    {nr: 11, originatesFrom: 3,    lastChanged: DateTime(2018,5,18)},
    {nr:  8, originatesFrom: 3,    lastChanged: DateTime(2018,5,13)},
    {nr: 10, originatesFrom: 3,    lastChanged: DateTime(2018,5,10)},
    {nr:  3, originatesFrom: null, lastChanged: DateTime(2018,5,3)}
]

复杂的排序规则如下：按照 originatesFrom 进行分组，每个组内部按照 lastChanged 降序排序，并且组按照第一个实例的 lastChanged 升序排序。最后，originateFrom 中为 null 的应该放在每个组的底部，与 originateFrom 中的 nr 匹配。（是的-这不是开箱即用的东西:-/）我已经尝试按 originatesFrom 对它们进行分组，但我真的需要将它们分开，以便可以单独对它们进行排序（我想？），而且我不确定是否明智使用 linq，而不是逐一处理所有对象，构建多个列表，最终将它们连接起来？好的一面是，很少会有真正的许多对象（因此效率可能不是太大的问题）。在排序之前，我不知道会有多少组，每个组会有多少对象。如果需要更好地解释排序规则，请告诉我！

- hasse

为什么 originatesFrom = 1 在 originatesFrom = 2 和 originatesFrom = 3 之间？ - sgmoore

@sgmoore: 由于第一项的 lastChanged - Jon Skeet

1个回答

网页内容由stack overflow 提供, 点击上面的

可以查看英文原文，
原文链接

- Jon Skeet · Accepted Answer

假设这实际上是在 List<T> 或类似的数据结构中（而不是通过 IQueryable<T> ，我们可能需要考虑查询是否可被翻译），我认为这样应该可以工作：

按 "originatesFrom 如果非空；否则按 nr" 分组
将每个分组转换为一个列表，按 originatesFrom （降序）和 lastChanged （降序）排序 - 这将使空的 originatesFrom 保持在最后
按 "first item 的 lastChanged"（升序）排序此列表
使用 SelectMany 压平结果

以下是针对你的示例数据的完整示例：

using System;
using System.Collections.Generic;
using System.Linq;

class Test
{
    static void Main()
    {
        var items = new[]
        {
             new { Number = 1, OriginatesFrom = (int?) null, LastChanged = new DateTime(2018,5,3) },
             new { Number = 2, OriginatesFrom = (int?) null, LastChanged = new DateTime(2018,5,3) },
             new { Number = 3, OriginatesFrom = (int?) null, LastChanged = new DateTime(2018,5,3) },
             new { Number = 4, OriginatesFrom = (int?) 1, LastChanged = new DateTime(2018,5,1) },
             new { Number = 5, OriginatesFrom = (int?) 2, LastChanged = new DateTime(2018,5,1) },
             new { Number = 6, OriginatesFrom = (int?) 1, LastChanged = new DateTime(2018,5,7) },
             new { Number = 7, OriginatesFrom = (int?) 1, LastChanged = new DateTime(2018,5,4) },
             new { Number = 8, OriginatesFrom = (int?) 3, LastChanged = new DateTime(2018,5,13) },
             new { Number = 9, OriginatesFrom = (int?) 1, LastChanged = new DateTime(2018,5,13) },
             new { Number = 10, OriginatesFrom = (int?) 3, LastChanged = new DateTime(2018,5,10) },
             new { Number = 11, OriginatesFrom = (int?) 3, LastChanged = new DateTime(2018,5,18 )}
        };

        var query = items
            .GroupBy(x => x.OriginatesFrom ?? x.Number)
            .Select(g => g.OrderByDescending(x => x.OriginatesFrom)
                          .ThenByDescending(x => x.LastChanged)
                          .ToList())
            .OrderBy(g => g.First().LastChanged)
            .SelectMany(g => g)
            .ToList();

        foreach (var item in query)
        {
            Console.WriteLine(item);
        }        
    }
}

输出结果与您所需的顺序相匹配：

{ Number = 5, OriginatesFrom = 2, LastChanged = 01/05/2018 00:00:00 }
{ Number = 2, OriginatesFrom = , LastChanged = 03/05/2018 00:00:00 }
{ Number = 9, OriginatesFrom = 1, LastChanged = 13/05/2018 00:00:00 }
{ Number = 6, OriginatesFrom = 1, LastChanged = 07/05/2018 00:00:00 }
{ Number = 7, OriginatesFrom = 1, LastChanged = 04/05/2018 00:00:00 }
{ Number = 4, OriginatesFrom = 1, LastChanged = 01/05/2018 00:00:00 }
{ Number = 1, OriginatesFrom = , LastChanged = 03/05/2018 00:00:00 }
{ Number = 11, OriginatesFrom = 3, LastChanged = 18/05/2018 00:00:00 }
{ Number = 8, OriginatesFrom = 3, LastChanged = 13/05/2018 00:00:00 }
{ Number = 10, OriginatesFrom = 3, LastChanged = 10/05/2018 00:00:00 }
{ Number = 3, OriginatesFrom = , LastChanged = 03/05/2018 00:00:00 }

不是LINQ太棒了吗？