WCF Cyclic Reference and can not solve with IsReference = true

Question

I know that this question have asked a million times. But my case, I weird and I don't understand. I got a cyclic reference and below are my code/configuration:

1. I have Entity as a base class for all entities.

   [DataContract(IsReference = true)]
   public class Entity : IEntity
   

2. Inherited class is like this, and created by Entity Framework 6

   [DataContract]
   public partial class User: Entity
   

   I have Group, User, GroupUser, GroupAdministrator. The name here is clear, I think. Group has GroupUser, User has GroupUser and GroupUser has Group & User.

3. In DbContext, proxy creation is turned off, lazy loading is on.

4. Error comes from this method, when I call it, I got a cyclic reference.

   public IList<Group> GetGroups()
   {
       return _groupRepository.GetAll(true, _ => _.GroupAdministrators.Select(__=>__.User), _ => _.GroupUsers.Select(__ => __.User));
   }
   

   I debug and the root cause is this: _ => _.GroupUsers.Select(__ => __.User)

   The _groupRepository.GetAll():

   public IList<T> GetAll(bool? isActive = true, params Expression<Func<T, object>>[] includes)
   {
       IQueryable<T> query = BrokerageSimulatorContext.Set<T>()
           .Where(_ => (isActive.HasValue && _.IsActive == isActive) || !isActive.HasValue);
       return includes.Aggregate(query, (current, includeProperty) => current.Include(includeProperty)).ToList();
   }
   

5. Why I need ? _ => _.GroupUsers.Select(__ => __.User) Because I need to include the Users of that group when list all groups.