Guard members

gistfile1.cs

public class Order : Aggregate {
  public Order Place(Customer customer, ...) {
    customer.GuardPlaceOrder();
    //...
  }
}

public class Customer Aggregate {
  private bool _hasMoreThanFiveOutstandingInvoices;

  internal void GuardPlaceOrder() {
    //The kind of rule(s) that go in here might change over time.
    Guard.Against(_hasMoreThanFiveOutstandingInvoices, PlaceOrderErrorCode.CustomerHasMoreThanFiveOutstandingInvoices);
  }
}

public static class Guard {
  public static void Against<TErrorCode>(bool assertion, TErrorCode errorCode) where TErrorCode : IConvertible {
    if(assertion) throw new OperationException<TErrorCode>(errorCode);
  }
}