LINQ sorting of property List in List of custom object

Question

I have two lists of objects, each object has the property Recommendations which itself is a list of 'Recommendation' objects. I want to sort the recommendation objects based on one of its properties. I have come up with this:

TPSqlORs.Where(x => x.Recommendations != null)
    .ToList()
    .ForEach(y => y.Recommendations.OrderBy(z => z.PointNumber));
SbmReportsORs.Where(x => x.Recommendations != null)
    .ToList()
    .ForEach(y => y.Recommendations.OrderBy(z => z.PointNumber));

But it causes no change at all to the original lists, which makes me suspect the ToList() is just making a copy and the sorting is happening on a copy which gets lost after execution. I searched along these lines but apparently whilst it does make a copy, the new list contains references to the original list elements, so surely it should be sorting them for both lists?

Please break your habit always to use ToList().ForEach. The ToList creates a fresh new collection from the first, so you are wasting cpu-cycles and even more memory just to be able to use List.ForEach instead of foreach. In this case you're even using it more superfluously since you can chain OrderBy directly behind Where. — Tim Schmelter
– Tim Schmelter, Commented Oct 2, 2013 at 9:08

Kamil Budziewski · Accepted Answer · 2013-10-02 09:04:56Z

4

OrderBy is not changing order of list. If you want changing order you should write:

y.Recommendations = y.Recommendations.OrderBy(z => z.PointNumber).ToList()

Read about OrderBy

answered Oct 2, 2013 at 9:04

Kamil Budziewski

23.1k14 gold badges88 silver badges107 bronze badges

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

3 Comments

Jon Skeet Over a year ago

You'd probably want a ToList call as well - to avoid re-ordering every time it's accessed, apart from anything else.

deed02392 Over a year ago

Should this be within the ForEach? If I replace as you've described I am told y does not exist in the context.

Kamil Budziewski Over a year ago

@deed02392 replace your y.Recommendations.OrderBy(z => z.PointNumber) with line above

Matthew Watson · Accepted Answer · 2013-10-02 09:37:25Z

2

Since Recommendations is in fact a List<Recommendation> you can sort it in-place using List.Sort():

item.Recommendations.Sort((lhs, rhs) => lhs.PointNumber.CompareTo(rhs.PointNumber));

This assumes that item is the object which contains the Recommendations that you want to sort.

If Recommendation elements in the list can be null, you can handle it like this:

item.Recommendations.Sort
(
    (lhs, rhs) =>
    (lhs == null || rhs == null)
    ? Comparer<object>.Default.Compare(lhs, rhs) 
    : lhs.PointNumber.CompareTo(rhs.PointNumber)
);

See List.Sort(Comparison<T> comparison) for details.

Note that List.Sort() is an unstable sort, while Enumerable.OrderBy() is a stable sort, but this is unlikely to make a difference to your case. You need to be aware of the difference though.

[Edit: I have incorporated the code from Jeppe Stig Nielsen's comment below; my thanks to him.]

edited Oct 2, 2013 at 9:37

answered Oct 2, 2013 at 9:17

Matthew Watson

111k12 gold badges179 silver badges301 bronze badges

3 Comments

deed02392 Over a year ago

Can you illustrate how this would best be done from your experience when item is within a List<item> and some of those items may have no Recommendations (null)

Jeppe Stig Nielsen Over a year ago

@deed02392 Do you mean the Recommendation itself can be null? Then you could do something like

(lhs, rhs) => lhs == null || rhs == null ? Comparer<object>.Default.Compare(lhs, rhs) : lhs.PointNumber.CompareTo(rhs.PointNumber)

. I assume that PointNumber is a non-nullable value type?

deed02392 Over a year ago

Correct @JeppeStigNielsen, thanks for this update. This answer is clearer IMO.

Colin Mackay · Accepted Answer · 2013-10-02 09:07:29Z

2

OrderBy will never change the order of the original list. It, along with the other LINQ methods, are pure functions. They don't modify the inputs and only create new outputs.

answered Oct 2, 2013 at 9:07

Colin Mackay

19.3k8 gold badges66 silver badges91 bronze badges

Comments

Anarion · Accepted Answer · 2013-10-02 09:15:41Z

0

Most of the LINQ expressions don't do changes to the source, they return a resulting collection.

edited Oct 2, 2013 at 9:15

answered Oct 2, 2013 at 9:08

Anarion

2,5443 gold badges33 silver badges45 bronze badges

1 Comment

Kamil Budziewski Over a year ago

Here it's IOrderedEnumerable

Collectives™ on Stack Overflow

LINQ sorting of property List in List of custom object

4 Answers 4

3 Comments

3 Comments

Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

3 Comments

3 Comments

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related