CODEWITHSUNDEEP
c#linq
Jeremy
Jeremy

Reputation: 46440

Using LINQ to merge a list of objects

I have two lists of objects, using Linq I would like to merge them, but where the two lists contain objects with the same key, I only want the one with the greatest LastUpdated Value.

I thought that I could somehow get a list grouping by key a with max(LastUpdated) then join back to the list joining on key and LastUpdated, but there must be a more efficient way...

List<MyObject> lstListA = new List<MyObject>;
List<MyObject> lstListB = new List<MyObject>;

public class MyObject
{
    public string Key {get;set;}
    public string Value {get;set;}
    public DateTime LastUpdated {get;set;}
}

Upvotes: 9

Views: 2772

Answers (3)

Amy B
Amy B

Reputation: 110221

Classic pick-a-winner.

IEnumerable<MyObject> query = lstListA
  .Concat(lstListB)
  .GroupBy(x => x.Key)
//now work with each group to pick a winner
  .Select(g => g.OrderByDescending(x => x.LastUpdated).First())

Upvotes: 2

Konamiman
Konamiman

Reputation: 50323

A bit comvoluted, but this seems to work as well:

var mergedItems = lstListA.Concat(lstListB);

mergedItems =
    (from item in mergedItems
    group item by item.Key into grp
    let sameKey = mergedItems.Where(obj => obj.Key == grp.Key)
    select sameKey.Where(obj => obj.LastUpdated == grp.Max(obj2 => obj2.LastUpdated)).Single()
).ToList();

Upvotes: 0

Jon Skeet
Jon Skeet

Reputation: 1503509

One option, using DistinctBy from MoreLINQ:

var query = lstListA.Concat(lstListB)
                    .OrderByDescending(x => x.LastUpdated)
                    .DistinctBy(x => x.Key);

Upvotes: 17

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