using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public class A

    {

        public bool myStr = true;

    }

    public static void Main()

    {

        List<A> List1 = new List<A>();

        List<A> List2 = new List<A>();

        List2.Add(new A(){ myStr = false});

        if(List2.All(x => x.myStr == ((bool?)(List1.FirstOrDefault()?.myStr)) ?? false))

        {

            Console.WriteLine("Hadfadfdfello World");

        }

        Console.WriteLine("Hello World");

    }

}