xUnit/Sources/basics/Model/Book.cs

namespace Model;

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

    public string Title
    { 
        get => title;
        set => title = value ?? "";
    }
    private string title = null!;

    public string Author
    { 
        get => author;
        set => author = value ?? "";
    }
    private string author = null!;

    public string Isbn
    { 
        get => isbn;
        set => isbn = value ?? "";
    }
    private string isbn = null!;

    public Person? Borrower { get; set; }

    public Book(long id, string title, string author, string isbn)
    {
        Id = id;
        Title = title;
        Author = author;
        Isbn = isbn;
    }
    public Book(string title, string author, string isbn)
        : this(0, title, author, isbn)
    {
    }

    public bool Equals(Book? other)
    {
        if (Id != 0)
            return Id == (other?.Id ?? 0);
        else
            return Title.Equals(other?.Title)
                && Author.Equals(other?.Author)
                && Isbn.Equals(other?.Isbn);
    }

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

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

    public override string ToString()
        => $"{Id} - {Title} ({Isbn}) // {Author}";
}