Validator.TryValidateProperty Not Working

我正在尝试实现Validator.TryValidateProperty，即使存在〔Required〕DataAnnotation，TryValidateProperties也会返回有效响应。

这是我的客户部分类别：

[MetadataType(typeof(Customer.Metadata))]
public partial class Customer : global::System.Data.Objects.DataClasses.EntityObject 
{
   ...
private sealed class Metadata
    {
        [Required]
        [SSNValidAttribute(ErrorMessage = "The SSN should be 9 numeric characters without any punctuation.")]
        [DisplayName("SSN")]
        public String SSN { get; set; }
...

这是返回True的代码：

...
var customer = new Customer();
            customer.SSN = "";
            var vc = new ValidationContext(customer, null, null);
            vc.MemberName = "SSN";
            var res = new List<ValidationResult>();
            var result = Validator.TryValidateProperty(customer.SSN, vc, res);
...

var customer = new Customer();
TypeDescriptor.AddProviderTransparent
(new AssociatedMetadataTypeTypeDescriptionProvider
    (customer.GetType()), customer.GetType());
customer.SSN = "";
var vc = new ValidationContext(customer, null, null);
vc.MemberName = "SSN";
var res = new List<ValidationResult>();
var result = Validator.TryValidateProperty(customer.SSN, vc, res);

TypeDescriptor.AddProviderTransparent
(new AssociatedMetadataTypeTypeDescriptionProvider
    (customer.GetType()), customer.GetType());