求Validator.TryValidateObject真相…该如何解决

    [MetadataType(typeof(Employee.EmployeeMetadata))]    public partial class Employee    {        internal sealed class EmployeeMetadata        {            [Display(Name = "用户编号", Order = 0, Description = "用户的唯一编号标志")]            [Required(ErrorMessage = "用户编号自动生成")]            [Editable(false)]            public global::System.Int32 PersonnelID;            [Display(Name = "名字", Order = 1, Description = "名字部分")]            [Required(ErrorMessage = "必须输入名字")]            [Editable(true)]            [StringLength(20)]            public global::System.String Firstname;            //...以下属性省略        }    }

            Employee mod = new Employee();            //...赋值赋值，从UI拿数据            ValidationContext vc = new ValidationContext(mod, null, null);            var validationResults = new List<ValidationResult>();            Validator.TryValidateObject(mod, vc, validationResults, true);            //就这里，无论我属性是什么他都没出错记录。            //比如我Firstname设置成了“我们是党的好儿女，我们坚决拥护党的领导和决策，现在已经很多字了……反正就是比20长很多很多……”都不会报错。