根据给定日期从列表中查找项目 [英] Find items from the list based on given date

问题描述

为了清楚显示，我有以下列表项.我可以形象地看到下面的小列表，该列表可能是几百行.

I have the following List item in order to display clearly. I could visualize the small list follows, that could be hundreds rows.

CourseId    ClassName    StartDate
--------    --------      --------  
12321       Math         08-25-2017 
14321       Math         08-25-2017   
32342       Physics      08-25-2017  
34345       Chemistry    08-25-2017 
25325       Math         01-25-2018
45329       Math         01-25-2018     
33325       Math         01-25-2018          
44345       Chemistry    01-25-2018    

我要传递ClassName和Date来检索相应的对象.我很难在LINQ中实现Date参数.以下实现仅排序并返回一项.

I have ClassName and Date to pass to retrieve the corresponding objects. I am having difficulty how to implement Date parameter into the LINQ. The following implementation sorts and returns only one item.

public List<Course> GetClassesByNameAndDate(string className, DateTime date, List<Courses> allCourses)
{
    List<Course> courses  = allCourses.Where( x=> x.ClassName == className && x.StartDate <= date ).OrderByDescending(x=> x.StartDate ).ToList();

}

如果我通过today date并且课程名称也通过Math，那么它应该从列表中返回25325,45329,33325 courseIDs对象.

If I pass today date and also course name as Math, then it should return me 25325,45329,33325 courseIDs objects from the list.

在另一个示例中，如果我给定日期01-01-2018，则它应该返回12321, 14321对象.因为01-01-2018早于25325,45329,33325开始日期，即01-25-2018.

In other example, if I give a date 01-01-2018, then it should return 12321, 14321 objects. Because 01-01-2018 is earlier than the 25325,45329,33325 startdate which is 01-25-2018.

推荐答案

我想我终于了解了要求.您不只想要一项.您也不希望列表包含startDate为<= date的所有项目.但是，您希望整个项目组的起始日期最接近(如果所有项目都具有相同的startDate ).您可以GroupBy日期:

I guess finally i have understood the requirement. You don't want only one item. You also don't want the list with all items where the startDate is <= date. But you want the whole group of items with the closest start-date(if all of them have the same startDate). You can GroupBy the date:

List<Course> courses  = allCourses
    .Where( x=> x.ClassName == className && x.StartDate <= date )
    .OrderByDescending(x=> x.StartDate)
    .GroupBy(x => x.StartDate)
    .FirstOrDefault()?.ToList() ?? new List<Course>();

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