Order a List by a arbitrary sort expression?

Question

Question:

I have a generic list, like this:

System.Collections.Generic.List<Question> myquestions = new System.Collections.Generic.List<Question>();

And I have a paging example, using a LINQ table acquried from database, doing this for paging:

var questions = context.Questions
    .OrderBy(sidx + " " + sord)
    .Skip(pageIndex * pageSize)
    .Take(pageSize);

Right now, for paging my populated from code list, I have:

var questionss = myquestions
    .OrderBy(x => x.Id)
    .Skip(pageIndex * pageSize)
    .Take(pageSize);

And what I want is being able to order "myquestions" by a string as in the above example. Is that possible?

Can you give examples of what sidx and sord would look like? Where do you get sidx and sord from. That is how do these change for every record? — Shiv Kumar, Mar 02 '11 at 08:07
http://stackoverflow.com/questions/41244/dynamic-linq-orderby ? — Marc Gravell, Mar 02 '11 at 09:44

Guffa · Answer 1 · 2011-03-02T08:32:21.500

You can chain the extension methods in separate statements thanks to the lazy execution, which means that you can add different sorting depending on a condition:

IOrderedQueryable<Question> sorted;
switch (sort) {
  case "Id":  sorted = myquestions.OrderBy(x => x.Id);
  case "Name": sorted = myquestions.OrderBy(x => x.Name);
  case "Size": sorted = myquestions.OrderBy(x => x.Size);
  case "Id Desc":  sorted = myquestions.OrderByDescending(x => x.Id);
  case "Name Desc": sorted = myquestions.OrderByDescending(x => x.Name);
  case "Size Desc": sorted = myquestions.OrderByDescending(x => x.Size);
  default: throw new NotImplementedException();
}
var questions = sorted.Skip(pageIndex * pageSize).Take(pageSize);

You can even add a secondary sorting (or as many as you like, after the first they are all the same):

IOrderedQueryable<Question> sorted;
switch (sort) {
  case "Id":  sorted = myquestions.OrderBy(x => x.Id);
  case "Name": sorted = myquestions.OrderBy(x => x.Name);
  case "Size": sorted = myquestions.OrderBy(x => x.Size);
  case "Id Desc":  sorted = myquestions.OrderByDescending(x => x.Id);
  case "Name Desc": sorted = myquestions.OrderByDescending(x => x.Name);
  case "Size Desc": sorted = myquestions.OrderByDescending(x => x.Size);
  default: throw new NotImplementedException();
}
switch (sort2) {
  case "Id":  sorted = sorted.ThenBy(x => x.Id);
  case "Name": sorted = sorted.ThenBy(x => x.Name);
  case "Size": sorted = sorted.ThenBy(x => x.Size);
  case "Id Desc":  sorted = sorted.ThenByDescending(x => x.Id);
  case "Name Desc": sorted = sorted.ThenByDescending(x => x.Name);
  case "Size Desc": sorted = sorted.ThenByDescending(x => x.Size);
}
var questions = sorted.Skip(pageIndex * pageSize).Take(pageSize);

Stefan Steiger · Accepted Answer · 2011-03-29T10:08:17.977

Bahaha, now I'm gonna cry:

Trying to implement dtb's solution, I saw the problem:

OrderByField<T>(IQueryable<T> q,

Returns IQueryable while inputting IQueryable... lol bs. So, the real question is how to convert List to be iQueryable.

Which, in fact is dead simple:

var list = new List<T>(); 
var queryable = list.AsQueryable();

then

queryable.OrderBy(sidx + " " + sord);

So the final solution is as trivial as:

myquestions.AsQueryable().OrderBy(sidx + " " + sord).Skip(pageIndex * pageSize).Take(pageSize);

Which also means Linq.Table implements IQueryable, which makes sense IMHO.

Edit:
Note: This requires reference to: Dynamic.dll (not System.Dynamic.dll) as well as

using System.Linq.Dynamic;

If you don't reference Dynamic.dll, or don't declare the using directive, you'll get an error on AsQueryable().

This is the best answer by miles. If you don't have System.Linq.Dynamic, just use nuget to get it. — MGOwen, Jul 13 '13 at 01:00

score 2 · Answer 3 · edited May 23 '17 at 11:45

2

Please check my answer to another question on how to build a custom OrderBy predicate using Expression from a given String.

edited May 23 '17 at 11:45

Community

1
1

answered Mar 02 '11 at 08:11

Cheng Chen

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I like it, but it still needs to be extended by using list.OrderByDescending(x => x.Id) when sortd is DESC. The problem is I don't quite understand it, YET (but I get the idea)... – Stefan Steiger Mar 02 '11 at 08:21
Thanks, the answer by dtb in your link is perfect. – Stefan Steiger Mar 02 '11 at 08:29
@Quandary: Well that's easy to implement in this solution, you don't need to modify too much. But the more dynamic, the more work(probably the slower). Follow @JSkeet's solution if you don't want it *too dynamic*. – Cheng Chen Mar 02 '11 at 08:29
@Quandary: LOL I don't know I should be happy or sad :( – Cheng Chen Mar 02 '11 at 08:30
Hmm, sad! Sad that I didn't realize the problem in those 10 seconds. See my answer. – Stefan Steiger Mar 02 '11 at 11:25

Jon Skeet · Answer 4 · 2011-03-02T08:20:23.373

1

One option is to create a dictionary from each possible ordering string to an appropriate delegate. For example:

Dictionary<string, Func<IEnumerable<Question>, IEnumerable<Question>> orderings =
    new Dictionary<string, Func<IEnumerable<Question>, IEnumerable<Question>>()
{
    { "Id",  questions => questions.OrderBy(x => x.Id) },
    { "Title",  questions => questions.OrderBy(x => x.Title) },
    { "Answer",  questions => questions.OrderBy(x => x.Answer) },
    { "Id Desc",  questions => questions.OrderByDescending(x => x.Id) },
    { "Title Desc",  questions => questions.OrderByDescending(x => x.Title) },
    { "Answer Desc",  questions => questions.OrderByDescending(x => x.Answer) },
};

Then you can find that mapping at execution time, and apply the function.

edited Mar 02 '11 at 08:20

answered Mar 02 '11 at 08:14

Jon Skeet

1,261,211
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I get the idea, but it's gonna create an awful lot of typing, plus it will be a 2d dictionary, because it also needs sort order ASC|DESC. Automatizing this would probably require reflection. – Stefan Steiger Mar 02 '11 at 08:17
@Quandary: You could always sort ascending and use `Reverse` if you want a descending list. I'd guess either the Linq query optimizer or the SQL server are smart enough to optimize that away and do a descending sort instead. – Niki Mar 02 '11 at 08:19
I'll remember that if I need very custom search order. – Stefan Steiger Mar 02 '11 at 09:15
@JonSkeet, does this still hold for you? – Rod Dec 09 '15 at 21:52
@Rod: What in particular? – Jon Skeet Dec 09 '15 at 22:09
Your dictionary answer. I'm trying to use the dynamic sortExpression string with linq. – Rod Dec 09 '15 at 22:29
@Rod: Well yes, as noted on the answer you accepted, you weren't doing anything with the result. – Jon Skeet Dec 09 '15 at 22:32
Ok thanks, I was just wondering if there was another concise way to do it. No biggie. – Rod Dec 09 '15 at 22:35
@Rod: It would have been helpful if you'd said that to start with. This is still the way I would personally do it. – Jon Skeet Dec 09 '15 at 22:36

score 1 · Answer 5 · answered Mar 02 '11 at 08:22

1

You could look at Dynamic Linq. IIRC it was a sample that shipped with VS2008, I'm not 100% sure if it's still included in VS2010.

answered Mar 02 '11 at 08:22

Niki

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Order a List by a arbitrary sort expression?

5 Answers5