实体框架有很多很多问题 [英] Entity framework many to many problem

问题描述

i首先在实体框架代码中向我的上下文添加新实体有问题

问题是当我想添加新作者预订或预订时作者说：

对象引用未设置为对象的实例。因为书的作者或作者的书是空的...而我设定的那个。

这里是我的代码：

 < span class =code-keyword> class 计划
 {
 静态  void  Main（ string  [] args）
 {
  var  db =  new  Source（）; 
发布商发布商= db.Publisher。单（p = >  p.Title ==  按）; 
 Book book =  new  Book（）{ISBN =   123424，Publisher = publisher，Title =   .NET简介 }; 
 book.Authors.Add（ new 作者（）{FirstName =   Bill，LastName =   Gates}）; 
 
 db.Books.Add（book）; 
 db.SaveChanges（）; 
} 
} 
 
  public   class 来源： DbContext 
 {
  public  Source（）： base （  Main）
 {
 
} 
 
 < span class =code-keyword> public  DbSet< Publisher>发布商{获取;  set ; } 
  public  DbSet< Author>作者{获取;  set ; } 
  public  DbSet< Book>图书{获取;  set ; } 
 
 受保护 覆盖  void  OnModelCreating（DbModelBuilder modelBuilder）
 {
 modelBuilder.Entity< Publisher>（）。HasKey（p = >  p.ID ）; 
 modelBuilder.Entity< Book>（）。Property（p = >  p.Title）.HasMaxLength（ 200 ）; 
} 
} 
 
  public   class  Publisher 
 {
  public   long  ID {获得;  set ; } 
 
  public   string 标题{获得;  set ; } 
  public   string 电子邮件{ get ;  set ; } 
  public   string 地址{ get ;  set ; } 
} 
 
  public   class 预订
 { 
  public   long  ID { get ;  set ; } 
 
  public   string 标题{获得;  set ; } 
  public   string  ISBN { get ;  set ; } 
 
  public 列表<作者>作者{获取;  set ; } 
  public  Publisher Publisher { get ;  set ; } 
} 
 
  public   class 作者
 { 
  public   long  ID { get ;  set ; } 
 
  public   string  FirstName {获得;  set ; } 
  public   string  LastName { get ;  set ; } 
 
  public 列表< Book>图书{获取;  set ; } 
} 

解决方案

Hello RedSakura，



I在这里纠正了你的代码。你错过了在作者类中创建书籍的新对象。

注意：我已经检查过静态数据，而不是数据库。这是完整的工作代码。

  var  db =  new 来源（）{作者= 新列表<作者>（），发布商= 新列表<发布商>（），图书= 新列表<图书>（）}; 
发布商发布商= 新发​​布商（）{标题=  按，地址=   India，Email =   [e-mail]，ID =  1 } ; 
 db.Publisher.Add（publisher）; 
 publisher = db.Publisher。 Single （p = >  p.Title == < span class =code-string> 按）; 
 
 Book book =  new  Book（）{ISBN =   123424，Publisher = publisher，Title =   .NET简介 ，Authors =  new  List< Author>（）}; 
 book.Authors.Add（ new 作者（）{FirstName =   Bill，LastName =   Gates}）; 
 db.Books.Add（book）; 

谢谢，

Imdadhusen

hi,
i have problem on adding new entities to my context in entity framework code first
problem is that when i want to add new author to book or book to author is says :
"Object reference not set to an instance of an object." because author of book or book of author is null... while i`v set that.
here is my code:

class Program
    {
        static void Main(string[] args)
        {
            var db = new Source();
            Publisher publisher = db.Publisher.Single( p=> p.Title == "Press");
            Book book = new Book() { ISBN = "123424", Publisher = publisher, Title = "Intro to .NET" };
            book.Authors.Add(new Author() { FirstName = "Bill", LastName = "Gates"});

            db.Books.Add(book);
            db.SaveChanges();
        }
    }

    public class Source: DbContext
    {
        public Source(): base("Main")
        {

        }

        public DbSet<Publisher> Publisher { get; set; }
        public DbSet<Author> Authors { get; set; }
        public DbSet<Book> Books { get; set; }

        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            modelBuilder.Entity<Publisher>().HasKey(p => p.ID);
            modelBuilder.Entity<Book>().Property(p => p.Title).HasMaxLength(200);
        }
    }

    public class Publisher
    {
        public long ID { get; set; }

        public string Title { get; set; }
        public string Email { get; set; }
        public string Address { get; set; }
    }

    public class Book
    {
        public long ID { get; set; }

        public string Title { get; set; }
        public string ISBN { get; set; }

        public List<Author> Authors { get; set; }
        public Publisher Publisher { get; set; }
    }

    public class Author
    {
        public long ID { get; set; }

        public string FirstName { get; set; }
        public string LastName { get; set; }

        public List<Book> Books { get; set; }
    }

解决方案

Hello RedSakura,

I have corrected your code here. you have missed creating new object of Books in Author class.
Note: I have checked with static data only not with Database. and this is complete working code.

var db = new Source() { Authors = new List<Author>(), Publisher = new List<Publisher>(), Books = new List<Book>() };
            Publisher publisher = new Publisher() { Title = "Press", Address = "India", Email = "[e-mail]", ID = 1 };
            db.Publisher.Add(publisher);
            publisher = db.Publisher.Single(p => p.Title == "Press");

            Book book = new Book() { ISBN = "123424", Publisher = publisher, Title = "Intro to .NET", Authors = new List<Author>() };
            book.Authors.Add(new Author() { FirstName = "Bill", LastName = "Gates" });
            db.Books.Add(book);

Thanks,
Imdadhusen

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