In some of my recent projects, I have modeled my domain layer following the design of that used in Amichai Mantinband's Buber Dinner course. To get the base models to work with EF Core, the AggregateRoot<TId, TIdType> class defines a new Id property of type AggregateRootId<TId> which hides the base Entity<TId>.Id property which is generically typed TId. While this solves the issue with EF Core, it introduces a new issue related to reflection.

When using instances derived from AggregateRoot<TId, TIdType> with certain libraries, such as Radzen Blazor Components and Mapster, a NullReferenceException is thrown because they are invoking Entity<TId>.GetHashCode() which in turn invokes the GetHashCode() method on Entity<TId>.Id. However, Entity<TId>.Id is never set.

The primary issue is that I don't really understand why Entity<TId>.Id is being referenced. I understand that the override of GetHashCode() is implemented in Entity<TId>, but the runtime should know that the underlying type of the object is AggregateRoot<TId, TIdType>, which hides the base Id property. I don't understand why the appropriate property is not being referenced. I could also define the GetHashCode() and other methods in the AggregateRoot<TId, TIdType> class, but that would defeat the entire purpose of inheriting from Entity<TId>.

I also fail to understand an issue related to one of the solutions. The solution being to modify the constructor of AggregateRoot<TId, TIdType> so that is passes the value down to the base constructor. As I'll explain in the next section, I was actually able to successfully solve this issue with this solution in this demo. However, when I tried this in one of my more complex projects, the value of the underlying Entity<TId>.Id property was still null at runtime. I am currently trying to figure out why it's not working in my other project and attempt to reproduce in this demo.

My first solution is to create DTOs which use the underlying type for the aggregate ids. If all I was doing was creating an API to expose publicly, this would be fine. However, since I am using these types in the presentation layers (e.g., Blazor, MAUI, etc.), using DTOs for this reason defeats the purpose of using strongly typed ids.

Update the constructor for AggregateRoot<TId, TIdType> so that it passes the value down to the base constructor as well.

protected AggregateRoot(TId id)
    : base(id) =>
    Id = id;

While this does appear to resolve the issue in this demo project, I have not been so lucky with one of my more complex projects. I will update soon with more information. To try and reproduce this solution not working, I will also be testing this with the Mapster library as well as EF Core. What I suspect is happening is that, because EF Core doesn't use the constructor, it only sets AggregateRoot<TId, TIdType>.Id when loading the entities from the database, thus leaving Entity<TId>.Id set to null.