code:

using System;
using System.Collections.Generic;

public static class MemoizedEvenChecker
{
    private static readonly Lazy<HashSet<int>> _evenNumbers = new Lazy<HashSet<int>>(() =>
    {
        var numbers = new HashSet<int>();
        for (int i = 0; i <= int.MaxValue; i += 2)
        {
            numbers.Add(i);
        }
        return numbers;
    });

    public static bool IsEven(int number)
    {
        return _evenNumbers.Value.Contains(number);
    }
}