如何通过linq查询对组中的元素进行排序,然后选择第一个？

Question

我有一组包含类型,日期和值的数据.

我想按类型分组,并且对于每组中的每组值,我想选择具有最新日期的那组.

这里有一些代码可以工作并给出正确的结果,但我想在一个linq查询中而不是在迭代中完成所有操作.任何想法我怎么能用纯粹的linq查询获得相同的结果...？

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

public class Program {

    public static void Main() {

        var mydata = new List<Item> {
            new Item { Type = "A", Date = DateTime.Parse("2016/08/11"), Value = 1 },
            new Item { Type = "A", Date = DateTime.Parse("2016/08/12"), Value = 2 },
            new Item { Type = "B", Date = DateTime.Parse("2016/08/20"), Value = 3 },
            new Item { Type = "A", Date = DateTime.Parse("2016/08/09"), Value = 4 },
            new Item { Type = "A", Date = DateTime.Parse("2016/08/08"), Value = 5 },
            new Item { Type = "C", Date = DateTime.Parse("2016/08/17"), Value = 6 },
            new Item { Type = "B", Date = DateTime.Parse("2016/08/30"), Value = 7 },
            new Item { Type = "B", Date = DateTime.Parse("2016/08/18"), Value = 8 },
        };

        var data = mydata.GroupBy(_ => _.Type);

        foreach (var thing in data) {

            #region 

            // How can I remove this section and make it part of the group by query above... ?          
            var subset = thing.OrderByDescending(_ => _.Date);
            var top = subset.First();

            #endregion

            Console.WriteLine($"{thing.Key} {top.Date.ToString("yyyy-MM-dd")} {top.Value}");

        }           
    }

    public class Item {

        public string Type {get;set;}
        public DateTime Date {get;set;}
        public int Value {get;set;}

    }   
}

// Output:
//   A 2016-08-12 2
//   B 2016-08-30 7
//   C 2016-08-17 6

Answer 1

使用select来获取FirstOrDefault(或First-有序下降,因为分组,你不会得到一个空的):

var data = mydata.GroupBy(item => item.Type)
                 .Select(group => group.OrderByDescending(x => x.Date)
                 .FirstOrDefault())
                 .ToList();

或者SelectMany与...一起Take(1)

var data = mydata.GroupBy(item => item.Type)
                 .SelectMany(group => group.OrderByDescending(x => x.Date)
                 .Take(1))
                 .ToList();