Question

I have a problem with these simple classes:

public class Thread
{
    public string Title { get; set; }
    public ICollection<Post> Posts { get; set; }
}

public class Post
{
    public DateTime Posted { get; set; }
    public string Text { get; set; }
}

I’d like a LINQ query that will return all Threads, sorted in latest post order. Assuming an Entity Framework DbContext with Threads and Posts, how to write it? Grouping is simple:

from t in Threads
group t.Posts by t into tg
select tg.Key;

But how to sort the Threads based on the latest Post.Posted?

Edit – Solution based on Jons answer:

from t in Threads
from p in t.Posts
group p by t into tg
orderby tg.Max(p => p.Posted) descending 
select tg.Key

Answer 1

You could use:

from t in Threads
group t.Posts by t into tg
orderby tg.Max(post => post.Posted) // Order by the latest post per thread
select tg.Key;

Obviously use descending if you want the threads ordered with the mostly-recently-posted thread first.