EF Core Meaning of constructor in Entity layer

Question

Hi everyone I just started work as .netcore developer. They are using .net core with ef core. I saw some entitys has got constructor. I wonder whats that means. It's be like

using Project.Entities.CompanyEntity;
using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations.Schema;
using System.Text;
namespace Project.Entities.Entity1
{
    [Table("E_Entity1")]
    public class EntityCategory : EntityBase
    {
        public EntityCategory()
        {   //What is does makes. It fill theese data when i call "get"  with DbContext
            OtherCategoryBranches = new List<OtherCategoryBranch>();
            AnotherCategories = new List<AnotherCategory>();
            
        }
        public Guid Id { get; set; }
        public string Name { get; set; }
        public string NormalizedName { get; set; }
        public string Description { get; set; }
        public Guid CompanyId { get; set; }

        public virtual Company Company { get; set; }
        public virtual List<OtherCategoryBranch> OtherCategoryBranches { get; set; }
        public virtual List<AnotherCategory> AnotherCategories { get; set; }   
      
    }
}

//in other entity,"Other" ,Related to EntityCategory
namespace Project.Entities.OtherCategory
{
    [Table("E_OtherCategoryBranches")]
    public class OtherCategoryBranches : EntityBase
    {
        public Guid Id { get; set; }

        public Guid EntityCategoryId { get; set; }         

        public virtual EntityCategory EntityCategory { get; set; } 
    }
}

2

In SSIS DatabaseDiagram the Entity1 table is

THe other related List's is for one to many right. And when call the get meton in DbContext it fill automaticly ?

It just initiates `one-to-many` or `many-to-many` [Navigation Property](https://learn.microsoft.com/en-us/ef/core/modeling/relationships?tabs=fluent-api%2Cfluent-api-simple-key%2Csimple-key) to avoid null references — Artur, Jan 07 '21 at 13:13

score 1 · Answer 1 · edited Jan 07 '21 at 13:46

Entity framework Core, "unfortunately" requires entities to have an empty constructor.

Let's say you'd have a class of Person for simplicity.

public class Person 
{
    // no constructor defined, will under the hood create a empty constructor for you 

    public string Name {get; set;}
    public virtual ICollection<Pet> Pets {get; set;}
}

public class Person 
{

    // no empty constructor defined for this class, will cause EF Core to throw an exception

    public Person(string name, IEnumerable<Pet> pets)
    {
      Name = name;
      Pets = pets;
    }

    public string Name {get; set;}
    public virtual ICollection<Pet> Pets {get; set;}
}

public class Person 
{
    
    // The empty constructor doesn't have to be public though, 
    // so to have some encapsulation and defend against object invariants, 
    //   you could have a protected empty constructor.

    protected Person()
    {

    }

    public Person(string name, IEnumerable<Pet> pets)
    {
      Name = name;
      Pets = pets;
    }

    public string Name {get; set;}
    public virtual ICollection<Pet> Pets {get; set;}
}

Hope this will give you some clarification.

as a final summary, the classes you've shared probably tries to avoid null reference exceptions on their list, and therefore adding constructors that ensures lists are newed up when a class gets instantiated.

this could also be achieved without actually declaring the constructor, as shown in these two examples.

public class Person 
{
    public Person()
    {
        Pets = new Collection<Pet>();
    }

    public string Name { get; set; }
    public virtual ICollection<Pet> Pets { get; set; }
}

// alternative:

public class Person 
{
    public string Name { get; set; }
    public virtual ICollection<Pet> Pets { get; set; } = new Collection<Pet>();
}

Yes I got a little. Do we need construcotr with new'ing list when we got related entity. Some others does not has constructor( will under the hood create a empty constructor for you ) But in this releted entity does not have empty constructor — pandakun, Jan 07 '21 at 13:21
you are thinking about the ``OtherCategoryBranches : EntityBase`` not having an empty constructor correct? it's probably because it doesn't have any lists as relations, so a constructor for newing up list relations is not necessary. — Bjarke Handsdal, Jan 07 '21 at 13:31
Yes Entitybase is a class but not having any constructor except created automaticly. Also I have one more question I updated my post — pandakun, Jan 07 '21 at 13:41

EF Core Meaning of constructor in Entity layer

1 Answers1