using System;

using System.Diagnostics.CodeAnalysis;

​

#nullable enable

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Hello World");

        InvestorAccount inv = new PrivateInvestorAccount();

        if (inv.IsRegistered)

        {

            if (inv is PrivateInvestorAccount pinv)

            {

                Person p = pinv.Holder;

            }

        }

    }

    public interface IHolder {}

    public class Person : IHolder {}

    public abstract class InvestorAccount

    {

        [MemberNotNullWhen(true, nameof(Holder))]

        public virtual bool IsRegistered => false;

        public virtual object? Holder { get; }

    }

    public class PrivateInvestorAccount : InvestorAccount

    {

        public override Person? Holder { get; }

    }

}