In the ClassifiedAd class, the Picture collection is not instantiated when doing a re-hydrate.

[paulvanbladel]

The ClassifiedAdd class has:

  public List<Picture> Pictures { get; private set; }

which is initialized in the following constructor:

 public ClassifiedAd(ClassifiedAdId id, UserId ownerId)
        {
            Pictures = new List<Picture>();
            Apply(
                new Events.ClassifiedAdCreated
                {
                    Id = id,
                    OwnerId = ownerId
                }
            );
        }

That's working fine but not in the re-hydrate scenario because the aggregate root doesn't touch that constructor when the event sourcing machinery is setting up via reflection the aggregate root.
A potential fix is to initialize the collection directly as follows:

public List<Picture> Pictures { get; private set; } = new List<Picture>();

[alexeyzimarev]

The best solution would be to use property initialiser. I will check the code as soon as I am done with Chapter 13.