Sort a List so a specific value ends up on top

Question

I have a class Offer which contains a filed Category.

I want all Offers of a specific category to appear on top, followed by all else.

I tried this, but to no avail, what would you recommend?

Offers = Offers.OrderBy(x => x.Category == "Corporate").ToList();

When you say "to no avail", what's the result of that expression? What's wrong with it? — Dan J, Dec 08 '11 at 20:01
take a look at http://stackoverflow.com/questions/728319/linq-orderby-against-specific-values — Glory Raj, Dec 08 '11 at 20:02
It doesn't sort properly, it seems to still be in the original order. — Wesley, Dec 08 '11 at 20:33

score 38 · Accepted Answer · answered Dec 08 '11 at 20:01

38

When you order by a boolean value false (0) comes before true (1). To get the elements that match the predicate first you should reverse the sort order by using OrderByDescending:

Offers = Offers.OrderByDescending(x => x.Category == "Corporate").ToList();

answered Dec 08 '11 at 20:01

Mark Byers

811,555
193
1,581
1,452

Uh, but this will reverse the order of everything else too. Try `OrderBy(x => x.Category != "Corporate")` instead. – BlueRaja - Danny Pflughoeft Dec 08 '11 at 20:06
3

+1. And perhaps adding a ThenBy(x => x.Category) to sort the remaining offers by Category name..? – Christoffer Lette Dec 08 '11 at 20:11
3

I'm hesitant to rely upon the ordering of false and true and that other developers would understand it. I'm inclined to make my intent absolutely clear, such as `.OrderBy(x => x.Category == "Corporate" ? 0 : 1)`. – Anthony Pegram Dec 08 '11 at 20:12
1

@BlueRaja-DannyPflughoeft the order of everything else will be undefined (if OrderBy uses an unstable sort, which I think it does). – phoog Dec 08 '11 at 20:40
Ah I see, I was misunderstanding the boolean argument – BlueRaja - Danny Pflughoeft Dec 08 '11 at 21:09

Olivier Jacot-Descombes · Answer 2 · 2018-02-06T13:13:34.317

The C# Language Specification 5.0 does not specify a byte representation for the true and false values. Therefore, it is better to not rely on the assumption that true is represented by 1. Also, the result of sorting by the Boolean expression x.Category == "Corporate" is not obvious, as true could be represented by a negative value as well. Therefore, I use a ternary operator to explicitly specify a sort value:

Offers = Offers
    .OrderBy(x => x.Category == "Corporate" ? 0 : 1)
    .ThenBy(x => x.Category)
    .ThenBy(x => x.Date) // or what ever
    .ToList();

Sort a List so a specific value ends up on top

2 Answers2

Linked