using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        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();

        Console.WriteLine(string.Join(Environment.NewLine, y.Select(pair => $"{pair.key} : {string.Join(", ", pair.list)}")));

    }

}