ASP.NET MVC 5条件验证

Question

我有以下表单模型：

public class Form {
    public string Field1;
    public string Field2;

    public Applicant PrimaryApplicant;
    public Applicant SecondaryApplicant;
}

public class Applicant {
    [Required(ErrorMessage = "Please enter a first name"), Display(Name = "First Name")]
    public string FirstName;
    [Required(ErrorMessage = "Please enter a last name"), Display(Name = "Last Name")]
    public string LastName;
}

主要申请人始终是必需的，而第二申请人是可选的，除非设置了任何字段。

我尝试使用IValidatbleObject接口和Validate方法实现它，但它不起作用。 （ModelState.IsValid仍然返回false）

    public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
    {
        var validationResult = new List<ValidationResult>();

        foreach (var property in TypeDescriptor.GetProperties(this).Cast<PropertyDescriptor>())
        {
            Validator.TryValidateValue(property.GetValue(this), validationContext, validationResult, property.Attributes.OfType<ValidationAttribute>());
        }

        var validationCount = TypeDescriptor.GetProperties(this)
                                        .Cast<PropertyDescriptor>()
                                        .Count(p => p.Attributes.OfType<ValidationAttribute>().Any());

        if (!IsPrimaryApplicant && validationResult.Count() == validationCount) validationResult.Clear();

        return validationResult;
    }

如何在此类情景中进行验证？