Linq查询以基于日期排序列表并获得不同

Question

I have a list as shown below:

List<MyObjects> list:
list[0] - Id: 1; RefId:101; Date:22/06/2016
list[1] - Id: 2; RefId:102; Date:24/06/2016
list[2] - Id: 3; RefId:101; Date:26/06/2016
list[3] - Id: 4; RefId:103; Date:28/06/2016

I need a linq query,which gets me distinct based on RefId having latest date, something like this:

List<MyObjects> newList:
list[0] - Id: 3; RefId:101; Date:26/06/2016
list[1] - Id: 2; RefId:102; Date:24/06/2016
list[2] - Id: 4; RefId:103; Date:28/06/2016

Here RefId-101 is repeated but its first date value is old, hence removed. Thanks in advance.

Answer 1

Using Linq Group by RefId then order each group by Date then take first out of each group :

list.GroupBy(x => x.RefId).Select(x => x.OrderByDescending(y => y.Date).FirstOrDefault());

OR using Query-Syntax:

List<MyObjects> results = (from x in list
                            group x by x.RefId into grp
                            select grp.OrderByDescending(y => y.Date).FirstOrDefault()
                            ).ToList();

Answer 2

You can use GroupBy to group by RefId , then sort this groups by Date and take the first ones:

var result = list.GroupBy(o => o.RefId)
      .Select(g => g.OrderByDescending(gx => gx.Date).First())
      .ToList();

Since you want the latest Date , you need to OrderByDescending .

Answer 3

I think it is best to avoid calling .FirstOrDefault() . Here's an alternative:

var newList =
    list
        .GroupBy(x => x.RefId)
        .SelectMany(x => x.OrderByDescending(y => y.Date).Take(1))
        .ToList();