C＃使用Linq计算每个日期的发生次数

Question

我有一个像这样的对象列表：

public class Entity
{
    public int Id { get; set; }
    public DateTime Date { get; set; }
}

| Id | Date                | 
| 1  | 2017-01-16 12:58:32 |
| 2  | 2017-01-16 11:36:01 |
| 3  | 2017-01-16 17:55:19 |
| 4  | 2017-01-19 13:19:40 |
| 5  | 2017-01-19 09:21:55 |

我想使用LINQ过滤这一点来计算白天的发生次数。所以结果会是这样的：

| Date       | Occurrences |
| 2017-01-16 |   3         | 
| 2017-01-17 |   2         | 

是否可以使用LINQ执行此操作？

Answer 1

您想使用GroupBy

var lst = new List<Entity>(); // populate with your data
var result = lst
    .GroupBy(x => x.Date.Date, x => x.Id)
    .Select(x => new { Date = x.Key, Occurrences = x.Count() });

Answer 2

尝试这样的事情

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.IO;
using System.Data;
using System.Xml;
using System.Xml.Linq;



namespace ConsoleApplication49
{
    class Program
    {

        static void Main(string[] args)
        {
            List<Entity> entities = new List<Entity>() {
                new Entity() { Id = 1, Date = DateTime.Parse("2017-01-16 12:58:32")},
                new Entity() { Id = 2, Date = DateTime.Parse("2017-01-16 11:36:01")},
                new Entity() { Id = 3, Date = DateTime.Parse("2017-01-16 17:55:19")},
                new Entity() { Id = 4, Date = DateTime.Parse("2017-01-19 13:19:40")},
                new Entity() { Id = 5, Date = DateTime.Parse("2017-01-19 09:21:55")}
            };

            var results = entities.GroupBy(x => x.Date.Date).Select(x => new { count = x.Count(), entities = x.ToList() }).ToList();
        }


    }
    public class Entity
    {
        public int Id { get; set; }
        public DateTime Date { get; set; }
    }



}

Answer 3

以下是两个解决方案：

1.第一个使用LINQ查询实体。

2.第二个使用多个LINQ函数对实体进行分组和计数。

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

namespace LinqExample
{
    class Program
    {
        public class Entity
        {
            public int Id { get; set; }
            public DateTime Date { get; set; }
        }
        static void Main(string[] args)
        {
            List<Entity> entities = new List<Entity>()
            {
                new Entity() { Id = 1, Date = DateTime.Parse("2017-04-14 21:02:37")},
                new Entity() { Id = 2, Date = DateTime.Parse("2017-04-14 21:03:42")},
            };

            var OccurencesPerDay = from entity in entities
                                   group entity by entity.Date.Date into g
                                   select new {Date = g.Key.Date, Occurences = g.Count()};
            // Above is more readable than, even though both are equal
            OccurencesPerDay = entities.
                GroupBy(ent => ent.Date.Date).
                Select(ents => new { Date = ents.Key.Date, Occurences = ents.Count()} );



            Console.WriteLine($"| Date | Occurences |");
            foreach (var occ in OccurencesPerDay)
            {
                Console.WriteLine($"| {occ.Date} | {occ.Occurences} |");
            }
        }
    }
}