Challenge 3: Chaining ?. and ?? Across a Two-Level Hierarchy — Possible Solution
====================================================================

Program.cs:

class Address
{
    public string? City;
}

class Customer
{
    public Address? Address;
}

Customer customerWithCity = new Customer { Address = new Address { City = "Springfield" } };
Customer customerNoAddress = new Customer { Address = null };

string city1 = customerWithCity?.Address?.City ?? "Unknown";
string city2 = customerNoAddress?.Address?.City ?? "Unknown";

Console.WriteLine(city1);
Console.WriteLine(city2);

Output:

    Springfield
    Unknown

Explanation:
For customerWithCity, every link in the chain is non-null, so
?.Address?.City resolves all the way through to the real city string,
and ?? never needs to supply its fallback. For customerNoAddress,
Address itself is null -- the ?. immediately short-circuits the rest
of the chain to null the moment it hits that null link, without
throwing a NullReferenceException, and the whole expression evaluates
to null. Because the overall result is null, ?? then supplies
"Unknown" as the fallback. No explicit `if (customer != null && ...)`
checks were written anywhere.

WHY THIS WORKS AS AN ANSWER
------------------------------
This builds the exact two-level nested structure the chapter's own
example uses and demonstrates both outcomes -- a fully-populated chain
and a chain broken partway through -- confirming ?. short-circuits
safely and ?? only kicks in when the chain actually resolves to null.