Challenge 3: An init-Only Property, Set Once — Possible Solution
====================================================================

Product.cs:

class Product
{
    public string Sku { get; init; } = "";
}

Working — Program.cs:

var product = new Product { Sku = "A1" };
Console.WriteLine(product.Sku);

Output:

    A1

Broken attempt:

product.Sku = "A2";   // attempting to reassign after construction

Representative compile error:

    Program.cs(2,9): error CS8852: Init-only property or indexer
    'Product.Sku' can only be assigned in an object initializer, or on
    'this' or 'base' in an instance constructor or an 'init' accessor.

Explanation:
`init` allows Sku to be set exactly once, via the object-initializer
syntax used when the Product instance is created (`new Product { Sku
= "A1" }`). Once construction is complete, the property behaves as if
it had no setter at all -- any assignment attempt after that point is
rejected at compile time, not merely discouraged. This is the same
mechanism used internally by C# 9's records, which rely on init-only
properties to give immutability without requiring every field to be
set entirely inside a hand-written constructor.

WHY THIS WORKS AS AN ANSWER
------------------------------
This demonstrates init allowing exactly one assignment via object-
initializer syntax, then reproduces the real CS8852 compile error the
chapter describes when a second assignment is attempted afterward.