Linq to select objects from two lists based on content

Question

I have two lists with objects. They are of the same length. Each object has an ID (which is represented in both lists), a change date and some data. I want to create a new list with the same ID's but with the object with the more recent change date, with an efficient Linq statement.

Example:

List 1:

ID: 1 ChangeDate 12:00 Data 1:1
ID: 2 ChangeDate 13:00 Data 1:2 <-
ID: 3 ChangeDate 14:00 Data 1:3

List 2:

ID: 1 ChangeDate 12:05 Data 2:1 <-
ID: 2 ChangeDate 12:55 Data 2:2
ID: 3 ChangeDate 14:10 Data 2:3 <-

Result:

ID: 1 ChangeDate 12:05 Data 2:1 
ID: 2 ChangeDate 13:00 Data 1:2
ID: 3 ChangeDate 14:10 Data 2:3

Cheers

Is the order of both lists the same?

willeM_ Van Onsem
– willeM_ Van Onsem

2017-09-07 19:44:49 +00:00
Commented Sep 7, 2017 at 19:44 — willeM_ Van Onsem
– willeM_ Van Onsem, Commented Sep 7, 2017 at 19:44
The order is not always the same.

Bagaboo
– Bagaboo

2017-09-07 19:46:03 +00:00
Commented Sep 7, 2017 at 19:46 — Bagaboo
– Bagaboo, Commented Sep 7, 2017 at 19:46

SwissCodeMen · Accepted Answer · 2021-05-11 05:49:53Z

2

If we suppose that you have a list which contains the lists you have mentioned:

var input = new List<Item> { list1, list2 };

where Item it the type of the objects that list1 and list2 include.

You can try the following:

var output = input.SelectMany(x=>x)
                  .GroupBy(x=>x.ID)
                  .Select(gr=>gr.OrderByDescending(x=>x.ChangeDate).First());

Essentially we project all the elements from the list in one list and then we group the items based on their ID. Then From each group we pick the most recent record.

If we have only the lists, we can concatenate them:

var input = list1.AddRange(list2);

and then try the following:

var output = input.GroupBy(x=>x.ID)
                  .Select(gr=>gr.OrderByDescending(x=>x.ChangeDate).First());

edited May 11, 2021 at 5:49

SwissCodeMen

5,03310 gold badges31 silver badges39 bronze badges

answered Sep 7, 2017 at 19:47

Christos

54.1k9 gold badges82 silver badges114 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Bagaboo Over a year ago

Excellent and elegant. Thank you!

Jeremi Over a year ago

Instead of putting both in an extra input list, you can just replace input.SelectMany(x => x) with list1.Union(list2).

Bagaboo Over a year ago

@Jeremi Even better. Thank you.

Collectives™ on Stack Overflow

Linq to select objects from two lists based on content

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related