public void MustMustValidateEntity() { CustomerValidator customerValidator = new CustomerValidator(); customerValidator.AddRule(x => x.RuleFor(y => y.Age).Must(y => y.HasValue && y > 35)); Assert.IsFalse(customerValidator.Validate(new Customer()).IsValid); Assert.IsFalse(customerValidator.Validate(new Customer { Age = 35 }).IsValid); Assert.IsTrue(customerValidator.Validate(new Customer { Age = 36 }).IsValid); }