LINQ FirstOrDefault返回超过1个结果

我正在尝试使用LINQ从列表中获取最旧的记录.我只想要TOP记录.看起来FirstOrDefault方法可以解决问题,但在LINQPad中测试我的查询.我得到的数据集不仅仅是一条记录..

有谁知道如何返回1条记录？

List schedule = new List()
{
    new Event(){eventID = 776734, eventName = "HGP", eventStartDate = Convert.ToDateTime("2011-09-01"), eventEndDate = Convert.ToDateTime("2011-09-18"), runningHours = 0.0000, runningStartStops = 0.0000},
    new Event(){eventID = 776735, eventName = "CI", eventStartDate = Convert.ToDateTime("2012-10-01"), eventEndDate = Convert.ToDateTime("2012-10-11"), runningHours = 0.0000, runningStartStops = 0.0000},
    new Event(){eventID = 776736, eventName = "HGP", eventStartDate = Convert.ToDateTime("2014-03-01"), eventEndDate = Convert.ToDateTime("2014-03-18"), runningHours = 0.0000, runningStartStops = 0.0000},
    new Event(){eventID = 779236, eventName = "CI", eventStartDate = Convert.ToDateTime("2015-11-10"), eventEndDate = Convert.ToDateTime("2015-11-23"), runningHours = 2774.3300, runningStartStops = 111.3400},
    new Event(){eventID = 779364, eventName = "MA", eventStartDate = Convert.ToDateTime("2017-03-11"), eventEndDate = Convert.ToDateTime("2017-04-04"), runningHours = 4176.0200, runningStartStops = 104.3400},
    new Event(){eventID = 779365, eventName = "CI", eventStartDate = Convert.ToDateTime("2019-04-05"), eventEndDate = Convert.ToDateTime("2019-04-13"), runningHours = 8054.6700, runningStartStops = 232.5100},
    new Event(){eventID = 779366, eventName = "HGP", eventStartDate = Convert.ToDateTime("2021-04-02"), eventEndDate = Convert.ToDateTime("2021-04-19"), runningHours = 13473.1300, runningStartStops = 389.3500},
    new Event(){eventID = 779367, eventName = "CI", eventStartDate = Convert.ToDateTime("2023-03-31"), eventEndDate = Convert.ToDateTime("2023-04-08"), runningHours = 18930.8900, runningStartStops = 520.0200}
};

var q = from evt in schedule
         where "MA,HGP".Contains(evt.eventName)
         group evt by evt.eventID into x
         select x.OrderByDescending(t => t.eventStartDate).FirstOrDefault();

//LINQPad method to show results     
q.Dump();

octavioccl.. 6

您需要在括号中关闭查询:

var q = (from evt in schedule
         where "MA,HGP".Contains(evt.eventName)
         group evt by evt.eventID into x
         select x.OrderByDescending(t => t.eventStartDate)).FirstOrDefault();

您的查询是选择每个已排序组的第一个元素.

更新

但是,如果您正在寻找最新的活动,为什么还需要分组呢？尝试将其更改group by为order by:

var q = (from evt in schedule
         where "MA,HGP".Contains(evt.eventName)
         order evt by evt.eventStartDate 
         select x).FirstOrDefault();

或者,正如@TimSchmelter在评论中所建议的那样,在对元素进行分组之前对其进行排序.

1> octavioccl..：

---

您需要在括号中关闭查询:

var q = (from evt in schedule
         where "MA,HGP".Contains(evt.eventName)
         group evt by evt.eventID into x
         select x.OrderByDescending(t => t.eventStartDate)).FirstOrDefault();

您的查询是选择每个已排序组的第一个元素.

更新

但是,如果您正在寻找最新的活动,为什么还需要分组呢？尝试将其更改group by为order by:

var q = (from evt in schedule
         where "MA,HGP".Contains(evt.eventName)
         order evt by evt.eventStartDate 
         select x).FirstOrDefault();

或者,正如@TimSchmelter在评论中所建议的那样,在对元素进行分组之前对其进行排序.

---

@ExceptionLimeCat:我在[我的第一条评论](http://stackoverflow.com/questions/34338919/linq-firstordefault-returing-more-than-1-result?noredirect=1#comment56419120_34338919)中提到了你的问题的原因.在group-by之前使用`evt.eventStartDate desc`命令,然后你不需要组中的一个,你也得到最新的组.目前您只是获得一个任意组,因为根本没有任何顺序.