Question

在EF6中添加子实体的正确方法是什么？为什么Parent.Children.Add（child）抛出NullReferenceException（Children集合为null）？如果Parent.Children集合至少有一个项目，则.Add（子）可以正常工作。我究竟做错了什么？这是我在MVC项目中的代码示例：

public class Parent
{
    public int ID { get; set; }
    public string Name { get; set; }
    public virtual ICollection<Child> Children { get; set; }
}

public class Child
{
    public int ID { get; set; }
    public string Name { get; set; }
    public int ParentID { get; set; }
    public Parent Parent { get; set; }
}

public class AppContext : DbContext
{
    public AppContext() : base("AppContext") {}

    public DbSet<Parent> Parent { get; set; }
    public DbSet<Child> Children { get; set; }
}

public class AppContextInitializer : System.Data.Entity.DropCreateDatabaseAlways<AppContext>
{
    protected override void Seed(AppContext context)
    {
        Parent parent = new Parent { Name = "John" };
        Child child = new Child() { Name = "Mary" };
        parent.Children.Add(child); // Doesn't work: System.NullReferenceException, Children==null

        Parent parent2 = new Parent { Name = "Robert" };
        Child child2 = new Child { Name = "Sarah", Parent=parent2 };
        context.Children.Add(child2); // Works.... even inserts the parent entity thru the child entity!!

        context.SaveChanges();
    }
}

Answer 1

您需要在Children的构造函数中初始化Parent导航属性：

public class Parent
{
   public int ID { get; set; }
   public string Name { get; set; }
   public virtual ICollection<Child> Children { get; set; }

   public Parent()
   {
     Children=new List<Child>();
   }
}

实体框架6 - 如何添加子实体？

1 个答案: