c# – LINQ FirstOrDefault返回1个以上的结果

发布时间：2020-12-15 23:29:31 所属栏目：百科来源：网络整理

导读：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.它看起来像FirstOrDefault方法,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？ ListEvent schedule = new ListEvent(){ new Event(){eventID = 776734,e

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.它看起来像FirstOrDefault方法,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<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"),new Event(){eventID = 776736,eventStartDate = Convert.ToDateTime("2014-03-01"),eventEndDate = Convert.ToDateTime("2014-03-18"),new Event(){eventID = 779236,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,eventStartDate = Convert.ToDateTime("2019-04-05"),eventEndDate = Convert.ToDateTime("2019-04-13"),runningHours = 8054.6700,runningStartStops = 232.5100},new Event(){eventID = 779366,eventStartDate = Convert.ToDateTime("2021-04-02"),eventEndDate = Convert.ToDateTime("2021-04-19"),runningHours = 13473.1300,runningStartStops = 389.3500},new Event(){eventID = 779367,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();

解决方法

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？尝试通过以下方式将组更改为订单：

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.

（编辑：李大同）

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