Sorting a Linq Query

Question

I cant for the life of me figure out why this List will not sort properly. Can someone point out what I am doing wrong?

List<WNBlogPost> posts = new List<WNBlogPost>();

IEnumerable<WNBlogPost> orderedPosts = (
    from p in posts
    select p
).OrderByDescending(c => c.ID);

foreach (WNBlogPost post in orderedPosts) {
    //output post to page
}

I am adding items to the posts list in loop, but the order they are added to the list is the same order they appear in after I add OrderByDescending().

Any idea what I am doing wrong?

Why would you use that syntax instead of posts.OrderByDescending(c => c.ID)? — Kirk Woll
– Kirk Woll, Commented Apr 19, 2012 at 18:04
Firstly what type is WNBlogPost.ID? Secondly you wouldn't happen to be looking at posts instead of orderedPosts by any chance? — Igby Largeman
– Igby Largeman, Commented Apr 19, 2012 at 18:07
@Duane: perhaps a silly question, but what does simply writing out post.ID look like? — user7116
– user7116, Commented Apr 19, 2012 at 18:15
so it turns out i am a complete idiot. the code was working properly, but a user control feeding my data was set to return the first 50 posts instead of the last 50. The sorting was working correctly, i was just sorting the wrong set. Thanks for all the help everyone — Duane
– Duane, Commented Apr 19, 2012 at 18:29

user7116 · Accepted Answer · 2012-04-19 18:05:02Z

12

LINQ queries (without external effort) do not introduce side-effects into the collections they operate on. Therefore, OrderByDescending only presents a sorted view of posts which you can access via orderedPosts.

// hat-tip: @JimSchubert
foreach (var post in orderedPosts)
{
    Frob(post); // these will be frobbed in descending order
}

If you would like to sort the actual list itself, you should use List<T>.Sort:

// utilize the Comparison<T> overload of List<T>.Sort to perform a
// descending sort based on the post ID
posts.Sort((post1, post2) => -post1.ID.CompareTo(post2.ID));

answered Apr 19, 2012 at 18:05

user7116

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1 Comment

Jim Schubert Over a year ago

Also, you then have to enumerate over orderedPosts for it to be useful :D

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