xUnit/Sources/basics/Model/Person.cs

using System.Collections.ObjectModel;
namespace Model;

public class Person : IEquatable<Person>
{
    public long Id { get; private init; }

    public string FirstName
    { 
        get => firstName;
        set => firstName = !string.IsNullOrWhiteSpace(value) ? value : "Jane";
    }
    private string firstName = null!;

    public string LastName
    { 
        get => lastName;
        set => lastName = value ?? "";
    }
    private string lastName = null!;

    public ReadOnlyCollection<Book> Books { get; private init; }
    private List<Book> books = new List<Book>();

    public Person(string firstName, string lastName)
        : this(0, firstName, lastName)
    { }

    public Person(long id, string firstName, string lastName)
    {
        Id = id;
        FirstName = firstName;
        LastName = lastName;
        Books = new ReadOnlyCollection<Book>(books);
    }

    public bool BorrowBook(Book book)
    {
        if(books.Contains(book))
            return false;
        books.Add(book);
        return true;
    }

    public bool ReturnBook(Book book)
    {
        return books.Remove(book);
    }

    public bool Equals(Person? other)
    {
        if(Id != 0)
            return Id == (other?.Id ?? 0);
        else
            return FirstName.Equals(other?.FirstName)
                && LastName.Equals(other?.LastName);
    }

    public override bool Equals(object? obj)
    {
        if(ReferenceEquals(obj, null)) return false;
        if(ReferenceEquals(this, obj)) return true;
        if(GetType() != obj?.GetType()) return false;
        return Equals(obj as Person);
    }

    public override int GetHashCode()
    {
        if(Id != 0)
            return (int)(Id % 7879);
        else
            return FirstName.GetHashCode() + LastName.GetHashCode();
    }

    public override string ToString()
        => $"{Id} - {FirstName} {LastName} (#books: {Books.Count})";
}