using System;

using System.Collections.Generic;

public class Program

{

    public class A

    {

        public int a1 {get; set;} = 0;

    }

    public class AA : A

    {

        public new int a1 {get; set;} = 123;

        public int a2 {get; set;} = 5;

    }

    public static void Main()

    {

        Dictionary<int, A> Items = new Dictionary<int, A>();

        Items[123] = new AA();

        A ttt = null;

        Items.TryGetValue(123, out ttt);

        Console.WriteLine("AA.a1: "+ ((AA)ttt).a1);

        Console.WriteLine("AA.a2: "+ ((AA)ttt).a2);

        Console.WriteLine("A.a1: "+ (ttt).a1);

    }

}