WPF Entity Framework, Custom ValidationRule for class.metadata.

Can't figure out how to do this, basically I want to create a custom Validation class which will allow me to define custom code like below:

namespace PhotoManagement.EntityData
{
    [MetadataTypeAttribute(typeof(User.UserMetadata))]
    public partial class User : BaseEntity
    {
        public void MetaSetUp()
        {
            TypeDescriptor.AddProviderTransparent(
                new AssociatedMetadataTypeTypeDescriptionProvider(typeof(User),
                typeof(UserMetadata)),
                typeof(User));
        }
        internal sealed class UserMetadata
        {
            [CompareString("Hash", ErrorMessage = "Passwords don't match!")] //Custom Validation
            public string passCheck { get; set; }
        }
    }
}

I have created a class which I believe is created correctly, however I can't seem to use it in the way I want to above, of course the below class doesn't do much at the moment, just testing to get it working first then will make it do what I want it to do.

namespace PhotoManagement
{
    public class CompareString : ValidationRule
    {
        private String _errorMessage = String.Empty;
        public string ErrorMessage
        {
            get { return _errorMessage; }
            set { _errorMessage = value; }
        }

        public override ValidationResult Validate(object value, CultureInfo cultureInfo)
        {
            var str = value as string;
            MessageBox.Show(str);

            if (String.IsNullOrEmpty(str))
            {
                return new ValidationResult(true, this.ErrorMessage);
            }

            return new ValidationResult(true, null);
        }
    }
}