LINQ orderby on日期字段按降序排列 [英] LINQ orderby on date field in descending order

问题描述

如何更改下面代码中的LINQ查询以按日期降序排列(最新，最早，最后)?

How can I change the LINQ query in the code below to sort by date in descending order (latest first, earliest last)?

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`

当前输出为:

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

我希望输出按以下顺序进行:

I want the output to in the following order:

may
april
march order

推荐答案

我认为Distinct()不能保证保持集合的顺序.

I don't believe that Distinct() is guaranteed to maintain the order of the set.

在转换为字符串之前，请先尝试提取匿名类型并对其进行区分/排序:

Try pulling out an anonymous type first and distinct/sort on that before you convert to string:

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);

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