Once you use Gee, is expected a set of warning at C compilation phase.

Use this code:

using Gee;

public class Empl : Object {
  public string name { get; set; }
}

public interface Col : GLib.Object, Gee.BidirList<Empl> {
  public abstract int length { get; }
}

public class MyHash : Gee.ArrayList<Empl>, Col {
  public int length { get { return size; } }
}

static void main () {
  var m = new MyHash ();
  var e = new Empl ();
  e.name = "Soe";
  m.add (e);
}

You will get:

gee-collections.vala.c: In function ‘my_hash_real_get_length’:
/home/despinosa/Proyectos/Tests/vala/gee-collections.vala.c:248:45: warning: passing argument 1 of ‘gee_abstract_collection_get_size’ from incompatible pointer type [-Wincompatible-pointer-types]
  _tmp0_ = gee_abstract_collection_get_size ((GeeCollection*) self);
                                             ^
In file included from gee-collections.vala.c:9:0:
jhbuild/install/include/gee-0.8/gee.h:1452:6: note: expected ‘GeeAbstractCollection * {aka struct _GeeAbstractCollection *}’ but argument is of type ‘GeeCollection * {aka struct _GeeCollection *}’
 gint gee_abstract_collection_get_size (GeeAbstractCollection* self);