public class BillingAddress {
  public Guid OrderId;
  public string StreetLine1;
  // etc.
}

public class Order { // this is the domain object
  private Guid _id;
  private Status _status;

  // note the behavior here - we throw an exception if it's not a valid state transition
  public Cancel() {
    if (_status == Status.Shipped)
      throw new InvalidOperationException("Can't cancel order after shipping.")
    _status = Status.Cancelled;
  }

  // etc...
}

public class Data.Order { // this is the persistence (EF) class
  public Guid Id;
  public Status Status;
}

public interface IOrderRepository {
  // The implementation of this will:
  // 1. Load the EF class if it exists or new it up with the ID if it doesn't
  // 2. Map the domain class to the EF class
  // 3. Save the EF class to the DbContext.
  void Save(Order order);
}