ComplexType entities are null if Skip() is applied to an IQueryable that uses a static method for Select mapping.

[vegardlarsen]

ComplexType entities are null if Skip() is applied to an IQueryable that uses a static method for Select mapping.

Steps to reproduce

async Task Main()
{
  var builder = new DbContextOptionsBuilder();
  
  // same results no matter which of these two providers you use
  //builder.UseSqlServer("Data Source=(localdb)\\MSSQLLocalDB;Initial Catalog=test;Integrated Security=True");
  builder.UseInMemoryDatabase("test");

  // setup consistent database state
  using (var context = new Context(builder.Options))
  {
    await context.Database.EnsureDeletedAsync();
    await context.Database.EnsureCreatedAsync();
    
    context.Tests.Add(new Test()
    {
      One = new ComplexTest { A = 42 }
    });
    
    await context.SaveChangesAsync();
  }  
  
  // test
  using (var context = new Context(builder.Options)) 
  {
    // create a queryable that maps the results using a static method
    var queryable = context.Tests.Select(t => Map(t));

    // Weird behavior starts here. 
    // If you run test case 1 first, it causes a NullReferenceException because the complex type
    // is null when the mapping happens. Since I can set a breakpoint and have it trigger in Map(), 
    // I assume this happens client-side in this case.
    // If you run test case 2 first (by commenting out test case 1), you get the desired result.
    // If you flip the order, and run 2 before 1, both of them gives you the correct result.
    // PS! .Dump calls are for comparing output in LINQpad.

    // Test case 1
    var skippedResults = await queryable.Skip(0).ToListAsync();
    skippedResults.Dump();

    // Test case 2
    var results = await queryable.ToListAsync();
    results.Dump();
  }
}

static int Map(Test x) {
  return x.One.A;
}

class Context : DbContext
{
  public Context(DbContextOptions options)
  : base(options)
  {
  }
  
  public DbSet<Test> Tests { get; set; }

  protected override void OnModelCreating(ModelBuilder modelBuilder)
  {
    base.OnModelCreating(modelBuilder);
    
    modelBuilder.Entity<Test>().OwnsOne(t => t.One);
  }
}

class Test 
{
  public int Id { get; set; }
  
  public ComplexTest One { get; set; }
}

[ComplexType]
class ComplexTest 
{
  public int A {get;set;}
}

I can't quite make heads or tails of this, but changing any of these things makes the problem disappear:

• Using a lambda to do the mapping directly: .Select(t => t.One.A)
• Reversing the order of the test cases.

Further technical details

EF Core version: 3.0.0
Database provider: Microsoft.EntityFrameworkCore.SqlServer and Microsoft.EntityFrameworkCore.InMemory
Target framework: .NET Core 3.0
Operating system: Windows 10
IDE: LINQpad 6 (and Visual Studio 2019)

[smitpatel]

Include seem to be ignored in first case.

[vegardlarsen]

@smitpatel According to the list of breaking changes, there shouldn't have been any changes to how ComplexType works, and this was working in 2.2 (without any Include calls).

[roji]

@vegardlarsen this is being investigated by @smitpatel and is probably a bug. The list of breaking changes only lists intentional breaking changes we made knowingly, but of course unintentional bugs slip through.

[smitpatel]

[ConditionalFact]
        public virtual void TEsting_issue_18140()
        {
            using (var context = CreateContext())
            {
                var query = context.Set<OwnedPerson>().Select(e => Map(e)).Skip(0).ToList();

            }
        }

        private static int Map(OwnedPerson p) => p.PersonAddress.Country.PlanetId;