日期排序优化答案

【问题标题】：Sorting optimization for date日期排序优化
【发布时间】：2018-08-11 05:04:16
【问题描述】：

I have been sorting a collection like this. i was wondering whether it can be optimized or not. please have a look:

//ordering elements i.e latest ToDate will be on top and if ToDate are same then latest FromDate will be in top
var posistions = (someModel.Positions.OrderByDescending(x => x.ToDate)
    .ThenByDescending(x => x.FromDate).Where(x => !x.ToDate.HasValue).ToArray())
    .Concat(someModel.Positions.OrderByDescending(x => x.FromDate)
    .ThenByDescending(x => x.ToDate).Where(x => x.ToDate.HasValue).ToArray());

someModel.Positions = posistions.ToArray();

【问题讨论】：

为什么要连接列表？也就是说，如果 Todate 没有值，那么它们应该排在最后，但是按 fromDate 排序？
首先我想，因为我是通过descend来订购ToDate，如果没有ToDate，它将移动到concat之后写入的块。你猜对了
第一个优化是不要将 10 个 LINQ 命令链接在一起。

标签： c# arrays linq sorting

【解决方案1】：

你可能只是做这样的事情。我不确定您需要订购什么极性，但它很容易改变

var positions = model.Positions
                     .OrderByDescending(x => x.ToDate.HasValue)
                     .ThenByDescending(x => x.ToDate)
                     .ThenByDescending(x => x.FromDate);

foreach (var pos in positions)
   Console.WriteLine(pos.ToDate+" : " + pos.FromDate);

输出

8/8/2018 5:34:49 AM : 8/8/2018 5:34:49 AM
8/7/2018 5:34:49 AM : 7/2/2018 5:34:49 AM
8/6/2018 5:34:49 AM : 6/22/2018 5:34:49 AM
null : 8/1/2018 5:34:49 AM
null : 7/22/2018 5:34:49 AM

Full Demo Here

【讨论】：

【解决方案2】：

见小提琴：https://dotnetfiddle.net/nNbT5D

对已接受答案的小调整：

.OrderBy(x => x.ToDate ?? DateTime.MinValue)
.ThenByDescending(x => x.FromDate);

这段代码足以复制结果。

使用此排序标准查看结果

From: 8/4/2018 7:35:00 AM ========= ToDate: 
From: 7/25/2018 7:35:00 AM ========= ToDate: 
From: 6/25/2018 7:35:00 AM ========= ToDate: 8/9/2018 7:35:00 AM
From: 7/5/2018 7:35:00 AM ========= ToDate: 8/10/2018 7:35:00 AM
From: 8/11/2018 7:35:00 AM ========= ToDate: 8/11/2018 7:35:00 AM

 result using question's sorting criteria below
From: 8/4/2018 7:35:00 AM ========= ToDate: 
From: 7/25/2018 7:35:00 AM ========= ToDate: 
From: 8/11/2018 7:35:00 AM ========= ToDate: 8/11/2018 7:35:00 AM
From: 7/5/2018 7:35:00 AM ========= ToDate: 8/10/2018 7:35:00 AM
From: 6/25/2018 7:35:00 AM ========= ToDate: 8/9/2018 7:35:00 AM

【讨论】：