C# Linq GroupBy Return Default Value Instead Of Null

I have the following list. I'm trying to split it into odd & even lists using GroupBy:

var x = new int[] { 2, 4, 6, 8, 10 };

var y = x
  .GroupBy(x => x % 2 == 0)
  .Select(x => x.ToList())
  .ToList();

This outputs a List<List<int>>, the problem is that it doesn't insert null groups into the list, so the list has only 1 item in it.

I'd like for the absent values to be inserted as an empty list, so that when I do:

var odds = y[1] // => empty list instead of out of range

I tried doing the following:

var y = x
  .GroupBy(x => x % 2 == 0)
  .Select(x => x.ToList() ?? new List<int>())
  .ToList();

Solution

I suggest using GroupJoin; we form all required groups' keys with Enumerable.Range(0, 2) and then join x to them:

var x = new int[] { 2, 4, 6, 8, 10 };

var y = Enumerable
  .Range(0, 2)
  .GroupJoin(x, 
             a => a, 
             b => b % 2, 
            (key, items) => (key : key, list : items.ToList()))
  .ToList();

https://dotnetfiddle.net/YpiBfm