我正在用 C# 开发一个项目。我有一个名为 ExamResult 的模型其中有一个名为 Date 的字段，定义为 String .

然后我定义如下

        var executionQuery = (from x in db.ExamResult
                     where x.Student.Equals(iStudent)
                     orderby x.Date
                     select *);

日期获取格式为 <YEAR>-<MONTH> 的值 像这样

2014-01
2013-04
2013-09

我想做的是创建一个表，该表获取所有返回日期的最小值，并创建一个表，该表与该最小日期的月数有差异。

例子:

当我们得到如上的结果时，我想得到下表(如果我们得到最小值是 2013-04)

9
0
5

我尝试执行以下操作，但我得到了 System.NotSupported异常

var dates = executionQuery.Select(x => int.Parse(x.Date.Substring(0,
4)) * 12 + int.Parse(x.Date.Substring(5, 2)) -
int.Parse(minDate.Substring(0, 4)) * 12 -
int.Parse(minDate.Substring(5, 2)));

你知道我该怎么做吗？

最佳答案

我会用一个小的 Func<TIn, TOut>委托(delegate)将您的字符串日期转换为 DateTimes，然后它们可以正确排序。

首先，一个将日期字符串转换为 DateTime 的简单方法对象:

// Split the string and instantiate new DateTime object to sort by later
Func<string, DateTime> getDate = s => {
    int[] dateParts = s
        .Split(new char[] {'-'})
        .Select(dp => int.Parse(dp))
        .ToArray();

    // Let's use the new DateTime(int year, int month, int day) constructor overload
    // dateParts[0] is the year and dateParts[1] is the month;
    // the magic number 1 below  is just a day to give to the DateTime constructor
    return new DateTime(dateParts[0], dateParts[1], 1); 
};

您的代码可能看起来像这样；我无法测试你的代码，所以这将取决于你如何让它工作:

注意我把你的Linq分开了查询并在 C# 中进行排序；这样您就可以随心所欲地从数据库中获取东西，然后订购这些元素。我希望这行得通；否则，你必须调用我 getDate Func两次 - 一次 orderby , 和一次 select ;我不喜欢那个选项。

// The select now builds an anonymous object; You can also create a new class, ExamResultWithDate,
// for example, that has all fields of ExamResult plus a DateTime field; OR you can just add that 
// property to the partial class generated by EF or Linq-to-Sql or whatever right on the ExamResult 
// entity.

var executionQuery = (from x in db.ExamResult
                     where x.Student.Equals(iStudent)
                     select new { Entity = x, ActualDate = getDate(x.Date) }); // note select * as in your OP doesn't compile :)

var orderedQuery = executionQuery
    .OrderBy(eq => eq.ActualDate)
    .Select(er => er.Entity); // gets you just the entities in this case and discards the dates

要获得不同的日期，只需对您的最短日期做一些简单的计算: 同样，这是您程序的伪代码；

// Let's get the minimum date and difference in months;
DateTime minDate = executionQuery
    .ToList()
    .Select(o => o.ActualDate)
    .Min();

// I am just using the dates here but you can easily use your entire entity or whatever you need
Dictionary<DateTime, int> datesWithMonthDifference = executionQuery
    .ToDictionary(
        eq => eq.ActualDate
        eq => ((eq.Year - minDate.Year) * 12) + eq.Month - minDate.Month // this formula calculates month difference as an integer 
    );

这是一个可以满足您需要的工作程序: 请注意，这只是一个示例，需要适合您的项目。

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

namespace DateTimeFromString
{
    class Program
    {

        static void Main(string[] args)
        {

            List<string> dates = new List<string>()
            {
                "2014-01",
                "2013-04",
                "2013-09"
            };

            // Split the string and instantiate new DateTime object to sort by later
            Func<string, DateTime> getDate = s => {
                int[] dateParts = s
                    .Split(new char[] {'-'})
                    .Select(dp => int.Parse(dp))
                    .ToArray();

                // Let's use the new DateTime(int year, int month, int day) constructor overload
                // dateParts[0] is the year and dateParts[1] is the month;
                // the magic number 1 below  is just a day to give to the DateTime constructor
                return new DateTime(dateParts[0], dateParts[1], 1); 
            };

            List<DateTime> sortedDates = dates
                .Select(d => getDate(d))
                .OrderBy(d => d)
                .ToList();

            Console.WriteLine(" Sorted Dates: ");
            sortedDates.ForEach(d => Console.WriteLine(d.Year.ToString() + " - " + d.Month.ToString()));

            // Let's get the minimum date and difference in months;
            DateTime minDate = sortedDates.Min();

            Dictionary<DateTime, int> datesWithMonthDifference = sortedDates
                .ToDictionary(
                    sd => sd,
                    sd => ((sd.Year - minDate.Year) * 12) + sd.Month - minDate.Month
                );

            Console.WriteLine();
            Console.WriteLine("Sorted dates with month difference:");

            foreach (var key in datesWithMonthDifference.Keys)
            {
                Console.WriteLine("{0} has difference of {1}", key, datesWithMonthDifference[key]);
            }
            Console.ReadKey();
        }
    }
}

我的测试程序的结果是这样的:

关于C#在几个月内有所不同？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/21515606/