EF core 3.1: should I initialize list navigation properties when using eager loading to load related entities?

Question

Consider the following EF core 3.1 data model:

public class Blog
{
    public int Id { get; set; }
    public string Name { get; set; }
    public List<Post> Posts { get; set; }
}

public class Post
{
    public int Id { get; set; }
    public string Author { get; set; }
    public DateTime Date { get; set; }
    public Blog Blog { get; set; }
    public int BlogId { get; set; }
}

public class BlogAppContext: DbContext
{
    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        optionsBuilder.UseSqlServer("Data Source=.;Initial Catalog=DB_BLOG_APP;Integrated Security=SSPI;MultipleActiveResultSets=True");
    }

    public DbSet<Post> Posts { get; set; }
    public DbSet<Blog> Blogs { get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Blog>()
            .HasMany(x => x.Posts)
            .WithOne(x => x.Blog)
            .HasForeignKey(x => x.BlogId)
            .IsRequired(); 
    }
}

Consider a scenario where I run the following query, by using eager loading:

static void Main(string[] args)
{
        using var context = new BlogAppContext();
        
        var blogs = context.Blogs.Include(x => x.Posts).ToList();

        foreach (var blog in blogs)
        {
            Console.WriteLine($"There are {blog.Posts.Count} posts");
        }
}

By looking at some examples, I've noticed that initalizing list navigation properties it's a common practice. In my case, this would lead to something like this:

public class Blog
{
    public int Id { get; set; }
    public string Name { get; set; }
    public List<Post> Posts { get; set; } = new List<Post<();
}

I'm asking whether this is really useful when querying via eager loading.

I have done a few tests and I have verified that the query showed above automatically creates an empty list for a blog having no posts .

Put another way, it seems that even if the Posts navigation property is not initialized to an empty list inside the Blog entity definition, a query using the eager loading doesn't care and does not return a null value for the Posts navigation property.

Is my understanding correct ?

If it does, what is the usefullness (if any) of initializing the Posts navigation property to an empty list when querying the database by using eager loading to load related entities ?

Answer 1

This is not useful and you don't have to do this because you are using eager loading(which is not very efficient if you load a lot of unnecessary data but this is another topic).

However, this is useful in two cases:

• If you don't use eager loading because Posts property may be null
• You are creating a new Blog object which is not yet saved in database and want to add a new Post into it. In this case Posts property must be initialized prior Posts.Add(post) is called because null ref unitialized exception will be thrown.