如何选择LINQ GroupBy子句中的最后一条记录

Question

I have the following simple table with ID , ContactId and Comment .

I want to select records and GroupBy contactId . I used this LINQ extension method statement:

Mains.GroupBy(l => l.ContactID)
 .Select(g => g.FirstOrDefault())
 .ToList()

It returns record 1 and 4 . How can I use LINQ to get the ContactID with the highest ID ? (ie return 3 and 6 )

Answer 1

You can order you items

Mains.GroupBy(l => l.ContactID)
.Select(g=>g.OrderByDescending(c=>c.ID).FirstOrDefault()) 
.ToList()

Answer 2

Use OrderByDescending on the items in the group:

Mains.GroupBy(l => l.ContactID)
    .Select(g => g.OrderByDescending(l => l.ID).First())
    .ToList();

Also, there is no need for FirstOrDefault when selecting an item from the group; a group will always have at least one item so you can use First() safely.

Answer 3

Perhaps selecting with Max instead of OrderByDescending could result into improving of performance (I'm not sure how it's made inside so it needs to be tested):

var grouped = Mains.GroupBy(l => l.ContactID);
var ids = grouped.Select(g => g.Max(x => x.Id));
var result = grouped.Where(g => ids.Contains(g.Id));

As I assume it could result into a query that will take MAX and then do SELECT * FROM ... WHERE id IN ({max ids here}) which could be significantly faster than OrderByDescending .

Feel free to correct me if I'm not right.

Answer 4

OrderByDescending

Mains.GroupBy(l => l.ContactID)
.Select(g=>g.OrderByDescending(c=>c.ID).FirstOrDefault()) 
.ToList()

is your best solution

It orders by the highest ID descending, which is pretty obvious from the name.

Answer 5

您可以像这样使用MoreLinq来获得更短的解决方案：

Main.OrderByDescending(i => i.Id).DistinctBy(l => l.ContactID).ToList();