LINQ orderby在日期字段中按降序排列

Question

如何在下面的代码中更改LINQ查询,按日期降序排序(最新的,最早的,最后一个)？

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


namespace Helloworld
{
    class MainClass
    {
        public static void Main (string[] args)
        {
            List<Envelops> env = new List<Envelops> ();
            Envelops e = new Envelops { ReportDate = DateTime.Now };
            env.Add (e);
            e = new Envelops { ReportDate = DateTime.Now.AddDays (5) };
            env.Add (e);
            e = new Envelops { ReportDate = new DateTime (2011, 3, 3) };
            env.Add (e);
            e = new Envelops { ReportDate = DateTime.Now };
            env.Add (e);

            foreach (Envelops r in env) {
                Console.WriteLine (  r.ReportDate.ToString("yyyy-MMM"));
            }

            var ud = (from d in env                  
                select  d.ReportDate.ToString("yyyy-MMM") ).Distinct();     

            Console.WriteLine ("After distinct");

            foreach (var r in ud) {
                Console.WriteLine (r);
            }

        }
    }

    class Envelops
    {
        public DateTime ReportDate { get; set; }
    }

}`enter code here`

目前的输出是:

2011-Apr
2011-May
2011-Mar
2011-Apr
After distinct
2011-Apr
2011-May
2011-Mar

我想按以下顺序输出:

may
april
march order

Answer 1

env.OrderByDescending(x => x.ReportDate)

Answer 2

我不相信Distinct()可以保证维持集合的顺序.

在转换为字符串之前,请尝试先取出匿名类型并对其进行distinct/sort:

var ud = env.Select(d => new 
                         {
                             d.ReportDate.Year,
                             d.ReportDate.Month,
                             FormattedDate = d.ReportDate.ToString("yyyy-MMM")
                         })
            .Distinct()
            .OrderByDescending(d => d.Year)
            .ThenByDescending(d => d.Month)
            .Select(d => d.FormattedDate);

Answer 3

这个声明肯定会帮助你：

env = env.OrderByDescending(c => c.ReportDate).ToList();