c# - LINQ FirstOrDefault 返回超过 1 个结果

Question

我正在尝试使用 LINQ 从列表中获取最旧的记录。我只是想要 TOP 本质上的记录。看起来 FirstOrDefault 方法可以解决问题，但在 LINQPad 中测试我的查询时。我得到的是一个数据集，而不仅仅是一条记录..

有人知道如何只返回1条记录吗？

List<Event> schedule = new List<Event>()
{
    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();

Answer 1

您需要在括号中结束您的查询:

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();

您的查询正在选择每个已排序组的第一个元素。

更新

但是，如果您只是在查找最新的事件，为什么还需要分组呢？尝试将 group by 更改为 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 在他的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。