EntityFramework Object Not Set to Instance of Object in Model Creation

Question

I have been struggling for a while now to convert a fairly large EntityFramework database created in model first to codefirst. I have a problem that I cant seem to resolve. I am getting an Object reference not set to an instance of an object with the following procedures on the call stack.

ModelConfiguration.Configuration.Types.EntityTypeConfiguration.Configure
ModelConfiguration.Configuration.ModelConfiguration.ConfigureEntities

to simplify the post I have created a test project that boils the problem down to its simplest form.

I have 3 classes

• a which has an has an optional b and an optional c
• b which has a collection of a's, and a colleciton of c's

• c which has an optional b and a collection of a's

  public class a
  {
      public int Id { get; set; }
      [Required]
      public string name { get; set; }
  
      public virtual b b { get; set; }
      [ForeignKey("b")]
      public int? b_Id { get; set; }
  
      public virtual c c { get; set; }
      [ForeignKey("c")]
      public int? c_Id { get; set; }
  }
  
  public class b
  {
      public int Id { get; set; }
      public string name { get; set; }
  
      public virtual ICollection<a> a_s { get; set; }
      public virtual ICollection<c> c_s { get; set; }
  }
  
  public class c
  {
      public int Id { get; set; }
  
      public virtual b b { get; set; }
      [ForeignKey("b")]
      public int? b_Id { get; set; }
  
      public virtual ICollection<a> a_s { get; set; }
  }
  
  public class MyContext : DbContext
  {
      public DbSet<a> a { get; set; }
      public DbSet<b> b { get; set; }
      public DbSet<c> c { get; set; }
  
      protected override void OnModelCreating(DbModelBuilder modelBuilder)
      {
          modelBuilder.Entity<a>()
              .HasOptional(m => m.b)
              .WithMany(m => m.a_s);
  
          modelBuilder.Entity<b>()
              .HasMany(m => m.c_s)
              .WithRequired(m => m.b);
  
          modelBuilder.Entity<c>()
              .HasMany(m => m.a_s)
              .WithOptional(m => m.c);
  
          base.OnModelCreating(modelBuilder);
      }
  
  }
  

  when I execute the code var a = from o in db.a select o, I get the error described above. There is absolutely no information on what is hapenning, so I really dont know where to turn. Can anyone help me solve this problem, as I really want to move away from Model First.

  namespace MvcApplication2.Controllers
  {
      public class HomeController : Controller
      {
          public ActionResult Index()
          {
              var db = new MyContext();
  
              var a = from o in db.a select o;
  
              return View();
          }
  
      }
  }
  

Answer 1

Try putting 'a' into local memory:

var a = from o in db.a.ToList() select o;

Answer 2

The mixup of fluent configuration and data annotation caused this problem. EF team should have handled this exception and given a meaningful error message.

Remove data annotations and use fluent configuration as follows

public class a
{
    public int Id { get; set; }

    [Required]
    public string name { get; set; }

    public virtual b b { get; set; }

    public int? b_Id { get; set; }

    public virtual c c { get; set; }

    public int? c_Id { get; set; }
}

public class b
{
    public int Id { get; set; }
    public string name { get; set; }

    public virtual ICollection<a> a_s { get; set; }
    public virtual ICollection<c> c_s { get; set; }
}

public class c
{
    public int Id { get; set; }

    public virtual b b { get; set; }

    public int? b_Id { get; set; }

    public virtual ICollection<a> a_s { get; set; }
}

public class NreContext : DbContext
{
    public DbSet<a> a { get; set; }
    public DbSet<b> b { get; set; }
    public DbSet<c> c { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Entity<a>()
            .HasOptional(m => m.b)
            .WithMany(m => m.a_s)
            .HasForeignKey(m => m.b_Id);

        modelBuilder.Entity<a>()
            .HasOptional(m => m.c)
            .WithMany(m => m.a_s)
            .HasForeignKey(m => m.c_Id);

        modelBuilder.Entity<c>()
            .HasOptional(m => m.b)
            .WithMany(m => m.c_s)
            .HasForeignKey(m => m.b_Id);

        base.OnModelCreating(modelBuilder);
    }
}