Challenge 3: An Extension Method Cannot Reach a Private Field — Possible Solution
====================================================================

BankAccount.cs:

public class BankAccount
{
    private decimal _balance = 100m;
}

Broken attempt — BankAccountExtensions.cs:

public static class BankAccountExtensions
{
    public static decimal GetBalanceDirectly(this BankAccount account)
    {
        return account._balance;   // attempting to read a private field
    }
}

Representative compile error:

    BankAccountExtensions.cs(5,24): error CS0122: 'BankAccount._balance'
    is inaccessible due to its protection level

Explanation:
Even though GetBalanceDirectly() is written with the this modifier
and is called with ordinary dot-syntax exactly like a real instance
method, it is NOT actually a member of BankAccount at all -- it's a
completely separate static method that merely happens to receive a
BankAccount as its first argument. Because it lives outside
BankAccount entirely, it is bound by the exact same access rules any
other outside code would be: _balance is private, so only code
written INSIDE the BankAccount class itself can read it. A genuine
instance method defined inside BankAccount would have no such
restriction.

WHY THIS WORKS AS AN ANSWER
------------------------------
This reproduces the exact private-member-access failure the chapter's
own limitation section describes, with the CS0122 error confirming
extension methods are bound by ordinary external access rules despite
their instance-method-like call syntax.