nhibernate Restrictions.Eq and null

public AbstractCriterion EqOrNull(string property, object value) {
    if (value == null)
        return Restrictions.IsNull(property);
    return Restrictions.Eq(property, value);
}

E.g.:

session.CreateCriteria<Advertisements>()
       .Add(EqOrNull(AdvertisementsProperties.City.ToString(), city));

Also see HHH-2951

session.CreateCriteria<Advertisements>()
       .Add(Expression.Or(
                 Expression.Eq("AdvName", "Cool Advertisement"),
                 Expression.IsNull("AdvName"))
        ).List<Advertisements>();