問題描述
根據最高日期從 IGrouping 中獲取項目 (Get an item from a IGrouping based on the highest date)
In a IEnumerable<Appointment> that contains appointments defined as:
class Appointment
{
public string Name { get; set; }
public DateTime LastModified { get; set; }
public bool IsCancelled { get; set; }
}
which in practice can be like:
"Jon S" | 25-02-2011 16:14:40 | true
"Jon S" | 25-04-2011 22:15:44 | false
"Marc G" | 15-11-2011 16:09:00 | true
"Marc G" | 21-12-2011 16:11:00 | false
"Marc G" | 20-12-2011 16:24:00 | true
"Eric L" | 13-06-2011 19:10:00 | false
"Reed C" | 13-04-2011 09:10:00 | true
"Reed C" | 13-06-2011 19:10:00 | false
"John G" | 04-01-2011 06:12:00 | true
"Jon S" | 25-03-2011 12:24:42 | true
This collection has to be downsized to a collection that contains only the appointments with the highest date LastModified of each distinct Name.
"Jon S" | 25-04-2011 22:15:44 | false
"Marc G" | 21-12-2011 16:11:00 | false
"Eric L" | 13-06-2011 19:10:00 | false
"Reed C" | 13-06-2011 19:10:00 | false
"John G" | 04-01-2011 06:12:00 | true
I am a bit confused how to do this. When I tried:
.GroupBy(n => n.Name)
.Select(d => d.Max(t => t.LastModified));
It seems to deliver the correct dates, but the whole entity is needed, what am I missing?.
UPDATE/note: This is a large collection with about 10.000 unsorted items, delivered by a service I cannot modify. Each distinct group has about 20 elements.
參考解法
方法 1:
.GroupBy(n => n.Name)
.Select(d => d.OrderByDescending(t => t.LastModified).First())
方法 2:
.GroupBy(appt => appt.Name, (key, group) =>
{
var maxDt = group.Max(appt => appt.LastModified);
return group.First(appt => appt.LastModified == maxDt);
});
Possibly better performance than previous solution
方法 3:
In Linq query syntax:
var last = from app in apps
orderby app.LastModified descending
group app by app.Name into grp
select grp.First();
Your version with max:
var lastApp = from app in apps
group app by app.Name into grp
select grp.First(last => last.LastModified == grp.Max( g => g.LastModified));
(by Caspar Kleijne、BFree、a553、Marino Šimić)