Entities for projection collection are tracked, but collection remains empty

[mnissl]

This issue is basically a follow up of #16636.

var entities = context.Products.Select(p => new ProductWrapper(p)
  {
    UserName = p.UserNavigation.Name,
    Components = new MyCustomList<ComponentWrapper>(
      p.Components.Select(c => new ComponentWrapper(c)
      {
        UserName = c.UserNavigation.Name
      }))
  });

While EF Core 3 Preview 7 (opposed to EF Core 2.2) does track all related entities (context.ChangeTracker.Entries().Count() returns the correct number), the Components collection remains empty!

Using ToList() instead of newing up manually a collection works in turn:

var entities = context.Products.Select(p => new ProductWrapper(p)
  {
    UserName = p.UserNavigation.Name,
    Components = p.Components.Select(c => new ComponentWrapper(c)
      {
        UserName = c.UserNavigation.Name
      }).ToList()
  });

But I need to use MyCustomList<T> instead of List<T>.

Further technical details

EF Core version: 3.0.0-preview7.19362.6
Database Provider: Microsoft.EntityFrameworkCore.SqlServer
Operating system: Windows 10 1903
IDE: Visual Studio 2019 16.2.0

[ajcvickers]

@smitpatel @maumar What is the expected behavior here when using a custom collection constructor?

[mnissl]

@smitpatel @maumar What is the expected behavior here when using a custom collection constructor?

I'd like to point out that MyCustomList<T> is not the issue; it also doesn't work with new List<T>():

var entities = context.Products.Select(p => new ProductWrapper(p)
  {
    UserName = p.UserNavigation.Name,
    Components = new List<ComponentWrapper>(
      p.Components.Select(c => new ComponentWrapper(c)
      {
        UserName = c.UserNavigation.Name
      }))
  });

Combining MyCustomList<T> with ToList() doesn't work either:

var entities = context.Products.Select(p => new ProductWrapper(p)
  {
    UserName = p.UserNavigation.Name,
    Components = new MyCustomList<ComponentWrapper>(
      p.Components.Select(c => new ComponentWrapper(c)
      {
        UserName = c.UserNavigation.Name
      }).ToList())
  });

[smitpatel]

Duplicate of #16318

When we invoke new List<>(...) the passed in parameter is empty list, which we populate afterwards. Hence the created list (custom or not) remains empty.