按Date.Date分组

Question

我正在努力做一个group by。我有以下列表：

 Date = 10/03 06:40 AM, Val = 10  
 Date = 10/03 08:55 PM, Val = 5  
 Date = 11/03 06:40 AM, Val = 5  
 Date = 11/03 10:50 AM, Val = 9  
 Date = 11/03 06:40 PM, Val = 14  

我想要这份名单：

 Date = 10/03, Val = 5  
 Date = 11/03, Val = 14

因此，列表按Date.Date分组，但Val取决于Max(d => d.Date)。

我是用foreach做的，但我很确定我们可以用LINQ (groupby，select)做得更好。有什么想法吗？

干杯

Answer 1

我相信你想要这样的东西

static T MaxBy<T, TKey>(this IEnumerable<T> source, Func<T, TKey> selector)
where TKey: IComparable<TKey> =>
    source.OrderByDescending(selector).First();

然后

from item in items
group item by item.Date.Date into byDate
select new
{
    Date = byDate.Key,
    byDate.MaxBy(item => item.Date).Val
} 

如果您不希望创建专用方法，当然也可以内联创建

from item in items
group item by item.Date.Date into byDate
let ordered = from item in byDate
              orderby item.Date descending
              select item.Val
select new
{
    Date = byDate.Key,
    Val = ordered.First() 
}

注:感谢mjwills，他指出了一个错误，我随后得以更正。

Answer 2

它应该是这样的：

var result = from d in list
             group d by d.Date.Date into grouped
             let max = grouped.Max(d => d.Date)
             select new {
                 Date = grouped.Key,
                 list.FirstOrDefault(c => c.Date == max)?.Val
             };

Answer 3

大致是这样的：

values
    .GroupBy(value => value.Date.Date)
    .Select(grouped => new
        {
            Date = grouped.Key,
            Val = values.First(value => value.Date == grouped.Date.Max()).Val
        });