Challenge 1: A Return() Method Using LINQ to Find the Loan First — Possible Solution
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Repository.cs (extended):

public class Repository<T>
{
    private readonly List<T> _items = new();
    public void Add(T item) => _items.Add(item);
    public IEnumerable<T> All() => _items;
    public IEnumerable<T> Where(Func<T, bool> predicate) => _items.Where(predicate);
    public void Remove(T item) => _items.Remove(item);
}

Program.cs:

var loans = new Repository<Loan>();
loans.Add(new Loan("001", "Alice", DateTime.Now.AddDays(7)));
loans.Add(new Loan("002", "Bob", DateTime.Now.AddDays(3)));
loans.Add(new Loan("003", "Carol", DateTime.Now.AddDays(-1)));

static void ReturnLoan(Repository<Loan> loans, string isbn, string borrower)
{
    var loan = loans.Where(l => l.Isbn == isbn && l.Borrower == borrower).FirstOrDefault();
    if (loan != null)
    {
        loans.Remove(loan);
        Console.WriteLine($"Returned: {loan}");
    }
}

ReturnLoan(loans, "002", "Bob");

foreach (var loan in loans.All())
    Console.WriteLine(loan);

Output:

    Returned: Loan { Isbn = 002, Borrower = Bob, DueDate = ... }
    Loan { Isbn = 001, Borrower = Alice, DueDate = ... }
    Loan { Isbn = 003, Borrower = Carol, DueDate = ... }

Explanation:
ReturnLoan uses Repository's own Where() (itself calling LINQ's real
Where() underneath, per csharp2-2/csharp2-7) to locate the specific
Loan record matching both isbn and borrower, then removes exactly
that record instance. Records compare by value (per csharp2-5), so
Remove() correctly identifies the matching record even though it's a
different object reference than the one originally added, as long as
every field matches.

WHY THIS WORKS AS AN ANSWER
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This uses the repository's own LINQ-backed Where() to find the
correct loan before removing it, demonstrating the generic
Repository<T> and LINQ working together exactly as the capstone's own
Repository.Where() method is designed to be used.