public PropertyInvariantAttribute(String nameOf, Contract.Comparison op, string value)
 {
     NameOf    = nameOf;
     Evaluator = ContractEvaluatorFactory.Create(op, value);
 }
 public PropertyInvariantAttribute(String nameOf, Contract.Constraint constraint)
 {
     NameOf    = nameOf;
     Evaluator = ContractEvaluatorFactory.Create(constraint);
 }