如何在遍历列表时比较 DateTime 对象？

Question

我正在尝试遍历包含两个字段的列表 (csv)；姓名和日期。列表中有各种重复的名称和各种日期。我试图推断列表中的每个名称，其中有多个相同名称的实例，对应的日期是最新的。

通过查看另一个答案，我意识到我需要使用 DateTime.Compare 方法，这很好，但我的问题是确定哪个日期更晚。一旦我知道这一点，我需要生成一个具有唯一名称和与之相关的最新日期的文件。

这是我成为新手的第一个问题。

编辑：

最初，我认为将 LatestDate 对象设置为不会显示在我的文件中的日期是“可以的”，因此将文件中的任何较晚日期设为 LatestDate。

这是我目前的编码：

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

namespace flybe_overwriter
{
class Program
{
    static DateTime currentDate;
    static DateTime latestDate = new DateTime(1000,1,1);
    static HashSet<string> uniqueNames = new HashSet<string>();

    static string indexpath = @"e:\flybe test\indexing.csv";
    static string[] indexlist = File.ReadAllLines(indexpath);
    static StreamWriter outputfile = new StreamWriter(@"e:\flybe test\match.csv");

    static void Main(string[] args)
    {

        foreach (string entry in indexlist)
        {

            uniqueNames.Add(entry.Split(',')[0]);

        }

        HashSet<string>.Enumerator fenum = new HashSet<string>.Enumerator();
        fenum = uniqueNames.GetEnumerator();

        while (fenum.MoveNext())
        {
            string currentName = fenum.Current;


            foreach (string line in indexlist)
            {
                currentDate = new DateTime(Convert.ToInt32(line.Split(',')[1].Substring(4, 4)), 
                                           Convert.ToInt32(line.Split(',')[1].Substring(2, 2)), 
                                           Convert.ToInt32(line.Split(',')[1].Substring(0, 2)));

                if (currentName == line.Split(',')[0])
                { 
                    if(DateTime.Compare(latestDate.Date, currentDate.Date) < 1)
                    {
                      //  Console.WriteLine(currentName + " " + latestDate.ToShortDateString() + " is earlier than " + currentDate.ToShortDateString());
                    }
                    else if (DateTime.Compare(latestDate.Date, currentDate.Date) > 1)
                    {
                     //   Console.WriteLine(currentName + " " + latestDate.ToShortDateString() + " is later than " + currentDate.ToShortDateString());
                    }
                    else if (DateTime.Compare(latestDate.Date, currentDate.Date) == 0)
                    {
                     // Console.WriteLine(currentName + " " + latestDate.ToShortDateString() + " is the same as " + currentDate.ToShortDateString());
                    }

                }
            }

        }


    }
}

}

任何帮助表示赞赏。 谢谢。

Answer 1

合而为一，在您的 Datetimes 上使用 Max() 函数，而不是进行自己的测试。

var result = indexList
        //"transform" your initial list of strings into an IEnumerable of splitted strings (string[])
        .Select(list => list.Split(','))
        //in this new List of string[], select the first part in text, select and Convert the second part in DateTime. 
        //We now have an IEnumerable of anonymous objects, composed of a string and a DateTime Property
        .Select(splittedList => new
                                    {
                                        text = splittedList[0],
                                        date = new DateTime(Convert.ToInt32(splittedList[1].Substring(4, 4)),
                                                            Convert.ToInt32(splittedList[1].Substring(2, 2)),
                                                            Convert.ToInt32(splittedList[1].Substring(0, 2)))
                                    })
        //group that new List by the text Property (one "entry" for each distinct "text"). 
        //GroupBy creates an IGrouping<out TKey, out TElement>, kind of special dictionary, with an IEnumerable<TResult> as "value" part 
        //(here an IEnumerable of our anonymous object)
        .GroupBy(textDateTimeList => textDateTimeList.text)
         //from this grouping, take the "key" (which is the "distinct text", and in the IEnumerable<anonymousObject>, take the Max Date. 
         //We now have a new List of anonymous object, with a string Property and a DateTime Property
        .Select(group => new
                             {
                                 stringField = group.Key,
                                 maxDateField = group.Max(dateField => dateField.date)
                             });