Decide on which constructor to call by convention

[ajcvickers]

EF may be able to call several of the constructors defined on an entity type. By convention, we should always choose the one with the most number of services that can map--similar to D.I. However, when mapping scalar properties, we could:

• Call the parameterless constructor (or the constructor with only services) if it is available. This would avoid any breaking change because this is the kind of constructor EF has always called.
• Call the constructor with the most scalar properties that match. The reasoning for this is that if a constructor that matches has been defined, then it could very well be intended that we call it. However, it may do extra validation or other things that are not needed/slow/harmful if called by EF

Note that if only one constructor is defined, then EF will use it--this is just about the case when multiple are defined.

[ajcvickers]

Initial decision: for scalar properties, match zero by default (avoids breaking change) then fewest to most (avoids ignoring field mapping configuration) and change using fluent API/annotations when available. Also, file an issue to consider changing this in the next breaking release.

[vanillajonathan]

It could also try to find a constructor by visibility, so that it first looks for a private constructor. So that a entity could be designed with a private constructor for use by EF, and a public constructor for use by developers for creating a new instance of the class.

I have to work around this using a a public static method together with a private constructor.

public class Blog
{
    private HashSet<Post> _posts = null!;

    // Private constructor for Entity Framework
    private Blog(int blogId, string url)
    {
        BlogId = blogId;
        Url = url;
    }

    public int BlogId { get; private set; }
    public string Url { get; private set; }
    public IReadOnlyCollection<Post> Posts => _posts.ToList().AsReadOnly();

    // Static method to create an instance with _posts set to empty instead of null.
    public static Blog Create(int blogId, string url)
    {
        var blog = new Blog(blogId, url);
        blog._posts = new HashSet<Post>();
        
        return blog;
    }
}

[weitzhandler]

I'm facing a similar issue related to readonly-properties. I'm using EF Core 3.

I'm trying to use readonly properties by taking advantage of constructors.
But in the other hand, I need to define a parameterless constructor as well.

Here's an example:

public class Contact
{
  public Contact() { }
  public Contact(string name) 
  {
    Name = name;
  }
   public string Name { get; }
}

When loading Contacts from store, EF uses the parameterless constructor instead of the other one that takes the property values.

Note: I've explicitly included the Name property in my model in the OnModelCreating routine.