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();

Solution 1

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();

Solution 2

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(); 

Comments

[Wesley]

It doesn't sort properly, it seems to still be in the original order.

[BlueRaja - Danny Pflughoeft]

Uh, but this will reverse the order of everything else too. Try OrderBy(x => x.Category != "Corporate") instead.

[Christoffer Lette]

+1. And perhaps adding a ThenBy(x => x.Category) to sort the remaining offers by Category name..?

[Anthony Pegram]

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).

[phoog]

@BlueRaja-DannyPflughoeft the order of everything else will be undefined (if OrderBy uses an unstable sort, which I think it does).

[BlueRaja - Danny Pflughoeft]

Ah I see, I was misunderstanding the boolean argument