實體框架6 - 如何添加子實體？

Question

在EF6中添加子實體的正確方法是什麼？爲什麼Parent.Children.Add（child）拋出NullReferenceException（Children集合爲null）？如果Parent.Children集合至少包含一個項目，則.Add（子項）將起作用。我究竟做錯了什麼？下面是從MVC項目我的代碼示例：

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

需要初始化在Parent構造的Children導航屬性：

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>(); 
    } 
}