Why navigation property doesn't populate when its class is private?

Question

I have this simple model:

class Parent
{
   public int Id { get; set; }

   public virtual ICollection<Child> Children { get; set; }
}

class Child
{
   public int Id { get; set; }
   public int ParentId { get; set; }

   public virtual Parent Parent { get; set; }
}

class MyContext : DbContext
{
    public DbSet<Parent> Parents { get; set; }
    public DbSet<Child> Children { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Child>().HasRequired(s => s.Parent).WithMany(s => s.Children).HasForeignKey(s => s.ParentId);

        base.OnModelCreating(modelBuilder);
    }
}

And when I use MyContext as below, I get null reference exception because child.Parent is null

var context = new MyContext();
var child = context.Children.First();
var parentId = child.Parent.Id;              // Parent == null

To solve this problem I have to change the access modifier of Parent and Child classes to be public.

Why is that required ? Or is this just a bug ?

Answer 1

That is not a bug, you are using a feature of Entity Framework called Lazy Loading, and to use it you need to meet some requirements that you can find in this link. One of those requirements is that your entity classes must be public. In that link you will find a proper explanation of why you should meet those requirements, but in summary your issue is because EF can't create a proxy class from your entity, due to that you can't use lazy loading. You already are meeting the main requirement for lazy loading which is your navigation properties must be virtual, but first you must meet the requirements that EF needs to create a proxy class.

As an additional resource I suggest you look this msdn page where you can find all the ways to load related entities using EF.