C# LINQ Orderby - How does true/false affect orderby?

Question

I was studying a bit of LINQ ordering as I have a list of Ids, and I need to order them sequentially. However, there are certain ids that need to take precedence over the standard ordering.

Given this C# code (which can be pasted into .NET Fiddle to test) the ordering works as I need it to, but I don't understand why a not (!) operator on a contains is giving me the correct ordering?

My expected ordering output is (5, 1, 2, 3, 4, 6, 7, 8, 9).

If I have a Contains in my ordering, shouldn't it give ordering priority to the rows that returned true? Instead it appears to give ordering priority to rows that return false.

using System.Linq; using System;  public class Program {   public static void Main()   {     var numbersToFilterBy = new [] {5, 11, 20};      var x = new [] {new XClass(){Id = 1}, new XClass(){Id = 9}, new XClass(){Id = 5}, new XClass(){Id = 3}, new XClass(){Id = 4}, new XClass(){Id = 2}, new XClass(){Id = 6}, new XClass(){Id = 8}, new XClass(){Id = 7}};      var trueData = (from data in x                    orderby !numbersToFilterBy.Contains(data.Id), data.Id                     select data).ToList();      foreach(var item in trueData){         Console.WriteLine(item.Id);   } }  public class XClass{     public int Id{get;set;}   } } 

What is the explanation as to why this happens?

Answer 1

The OrderBy method will sort items in ascending order by default. Now, given that the numeric representation of a boolean is:

• false = 0
• true = 1

false values will naturally come first. If you want to reverse the order just use the descending keyword:

var trueData = (from data in x                orderby numbersToFilterBy.Contains(data.Id) descending, data.Id                 select data).ToList(); 

Answer 2

Basically, false is earlier than true... think of them as false=0, true=1. This is in-keeping with the documentation for bool.CompareTo(bool).

If you want to prioritize "true" values to the start, just use OrderByDescending instead.