.Net中集合排序还可以这么玩

时间：2022-09-03 19:06:32

背景：

.Net中集合排序还可以这么玩 
public class StockQuantity
    {
        public StockQuantity(string status, DateTime dateTime, int quantity)
        {
            Status = status;
            DateTime = dateTime;
            Quantity = quantity;
        }

        public string Status { get; set; }
        public DateTime DateTime { get; set; }
        public int Quantity { get; set; }

}
.Net中集合排序还可以这么玩


该对象，主要有三个字段，现在的业务需求是，取到了一个类型为List<StockQuantity>集合StockQuantities，需要对该集合进行三次排序，排序规则及优先级如下：
1.    Status为空的排在后面，不为空的排在前面，不关心Status的内容，只关心Status是否为空。
2.    DateTime升序排序。
3.    Quantity升序排序。

小白我的做法：

 

我只知道可以对集合用OderBy排序，对以上三条规则，所以设计思路如下。

1.    StockQuantities.OrderBy(u=>u.Status)
错误，
该排序得规则不仅仅会考虑Status是否为空，还会考虑Status的内容。
如果Status是[“b”,”c”,null,”d”]，那么排序结果是[null,“b”,”c”,”d”]。
而我们要的结果是[“b”,”c”,”d” ,null]  （直接把null的丢到最后，别的不动）
怎么办？

暂时不知道，先不管

2.    对DateTime进行升序排序，这简单
StockQuantities.OrderBy(u=>u.DateTime)
半对！
为什么半对，看下面

3.    在排序2的前提下，用OrderBy,也就是StockQuantities.OrderBy(u=>u.DateTime).OrderBy(u=>u.Quantity)
错误！
以上表达式等同于下面两条的表达式：

StockQuantities = StockQuantities.OrderBy(u=>u.DateTime)
StockQuantities = StockQuantities.OrderBy(u=>u.Quantity)

所以第一条代码就是废代码，最终排序还是以Quantity进行排序的。
虽然我是小白，但我还是明白这样是错误的，所以我的做法是

.Net中集合排序还可以这么玩 
stockQuantities = stockQuantities.OrderBy(u => u.DateTime).ToList();

            foreach (var dateOrder in stockQuantities)
            {
                var datetimeOrderBy = stockQuantities.Where(u => u.DateTime.Date == dateOrder.DateTime.Date) .OrderBy(u => u.Count);

                foreach (var countOrder in datetimeOrderBy)
                {
                    if (countOrder.OutPut == false)
                    {
                        Console.WriteLine($"{countOrder.Status}-{countOrder.DateTime}-{countOrder.Count}");
                        countOrder.OutPut = true;
                    }
                    
                }
            }
            Console.ReadKey();
.Net中集合排序还可以这么玩


采用双层循环，先取到按时间排序的数据 dateOrder,再去和该条数据在同一天的所有数据并对Quantity进行排序，为了防止重复的输出，我同时给StockQuantity对象加上了Output属性，当该属性为false为，则输出该对象的内容，并把Output属性设为true，这样就不会重复输出了，而且实现了先对DateTime排序，再对Quantity进行排序。
So Easy！！
然而，当开心地把这样的代码提交之后，却被同事狠狠地鄙视了，说到：“什么烂代码啊！”然道还有比这更好的代码？

给同事倒了一杯茶，点了一根烟，虚心请教。

大佬做法：


同事给我讲了两招，分别是条件排序、多级排序。

什么是条件排序，怎么用？

1.    StockQuantities.OrderBy(u=>u.Status==null)
这就是条件排序，可是咋一看，给人一种是把Status为空的排前面，不为空的排后面的错觉。
其实不然，我们看到OrderBy里面的一个返回值为bool类型的表达式，该排序先排结果为0（false）的，再排结果为1(true)的。这种排序只考虑返回的bool值，不考虑参数的具体值，所以姑且称它为条件排序。
完全符合排序规则1的要求。

什么是多级排序，怎么用？


2.    利用我上面我的代码排序虽然可以实现先排DateTime,再排Quantity,但是该算法的时间复杂度的n*n，而且给StockQuantity添加了output字段，明显是不科学的。
然而，连续地使用多个OrderBy最终只会生效最后一个OrderBy,天无绝人之路，所以这个时候应该使用ThenBy！！
使用ThenBy可以讲以上的三条排序规则简化如下：
stockQuantities = stockQuantities.OrderBy(u => u.Status==null).ThenBy(u => u.DateTime).ThenBy(u => u.Quantity).ToList();
即可完美地实现再前一个排序前提下进行二级排序。

优化后的完整代码如下：

.Net中集合排序还可以这么玩 
using System;
using System.Collections.Generic;
using System.Linq;

namespace OrderBy
{
    class Program
    {
        static void Main(string[] args)
        {
            var stockQuantities = new List<StockQuantity>()
            {
                new StockQuantity("正常品",new DateTime(2017,4,16),12 ),
                new StockQuantity("正常品",new DateTime(2017,4,17),15 ),
                new StockQuantity("残次品",new DateTime(2017,4,16),10 ),
                new StockQuantity("残次品",new DateTime(2017,4,17),8 ),
                new StockQuantity(null,new DateTime(2017,4,18),8 ),
            };

            stockQuantities = stockQuantities.OrderBy(u => u.Status==null).ThenBy(u => u.DateTime).ThenBy(u => u.Quantity).ToList();

            foreach (var stockQuantity in stockQuantities)
            {
                Console.WriteLine($"{stockQuantity.Status}-{stockQuantity.DateTime}-{stockQuantity.Quantity}");
            }

            Console.ReadKey();
        }
    }

    public class StockQuantity
    {
        public StockQuantity(string status, DateTime dateTime, int quantity)
        {
            Status = status;
            DateTime = dateTime;
            Quantity = quantity;
        }

        public string Status { get; set; }
        public DateTime DateTime { get; set; }
        public int Quantity { get; set; }

    }
}
.Net中集合排序还可以这么玩

简单的一个排序优化，就把程序的时间复杂度从N*N降低到了N，所以在这里把这两种排序技巧分享出来，希望对不会的同学有所帮助。

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