C#中结构体为值类型，类是引用类型

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApp1
{

    class BookClass
    {
        private string title;
        private string author;
        private string subject;
        private int book_id;
        public void setValues(string t, string a, string s, int id)
        {
            title = t;
            author = a;
            subject = s;
            book_id = id;
        }
        public void display()
        {
            Console.WriteLine("Title : {0}", title);
            //Console.WriteLine("Author : {0}", author);
            //Console.WriteLine("Subject : {0}", subject);
            //Console.WriteLine("Book_id :{0}", book_id);
        }

    };

    struct Bookstruct
    {
        private string title;
        private string author;
        private string subject;
        private int book_id;
        public void setValues(string t, string a, string s, int id)
        {
            title = t;
            author = a;
            subject = s;
            book_id = id;
        }
        public void display()
        {
            Console.WriteLine("Title : {0}", title);
            //Console.WriteLine("Author : {0}", author);
            //Console.WriteLine("Subject : {0}", subject);
            //Console.WriteLine("Book_id :{0}", book_id);
        }

    };


    public class Class2
    {
        public static void Main(string[] args)
        {

            Bookstruct Book1 = new Bookstruct(); /* 声明 Book1，类型为 Books */
            Bookstruct Book2 = new Bookstruct(); /* 声明 Book2，类型为 Books */

            Book1.setValues("Book1", "Nuha Ali", "C Programming Tutorial", 6495407);

            Book2.setValues("Book2","Zara Ali", "Telecom Billing Tutorial", 6495700);

            Book1.display();
            Book2.display();

            Book2 = Book1;
            Book2.setValues("Book2", "Zara Ali", "Telecom Billing Tutorial", 6495700);
            Console.WriteLine("Book1.display():");
            Book1.display();

            Console.WriteLine("=====================");
            BookClass book1 = new BookClass();
            BookClass book2 = new BookClass();
            book1.setValues("book1", "Nuha Ali", "C Programming Tutorial", 6495407);
            book2.setValues("book2", "Zara Ali", "Telecom Billing Tutorial", 6495700);
            book1.display();
            book2.display();

            book2 = book1;
            book2.setValues("book2", "Zara Ali", "Telecom Billing Tutorial", 6495700);
            Console.WriteLine("book1.display():");
            book1.display();

            Console.ReadKey();

        }
    }
}
/*
Title : Book1
Title : Book2
Book1.display():
Title : Book1
=====================
Title : book1
Title : book2
book1.display():
Title : book2
*/