title: Conditional Data Annotation and dropdown viewmodel not validating properly
body: <p>I have these viewmodels that I have combined to be used in a form. However When I validate the form and leave the Email, PhoneNumber and Method dropdown list blank, no validation appear and the ModelState is valid in the debug section.</p>
  
  <p>The dropdown list doesn't seem to return any validation errors as well for some reason. When I debug, the input is null which is on purpose but it doesn't show any errors back</p>
  
  <p>These are the viewmodels:</p>
  
  <pre><code>public class UnsubscribeContactDetailsViewModel
  {
      [Display(Name = "Phone Number")]
      public string PhoneNumber { get; set; }
      public string Email { get; set; }
  }
  
  public class MethodsDropdownListViewModel
  {
      public string Method { get; set; }
      public IEnumerable&lt;SelectListItem&gt; Methods { get; set; }
  }
  
  public class RegisterUnsubscribeViewModel : IValidatableObject
  {
      public string Id { get; set; }
      [Required]
      public UnsubscribeContactDetailsViewModel ContactDetails { get; set; }
      public MethodsDropdownListViewModel MethodsDropdownList { get; set; }
      public DateTime CreatedOn { get; set; }
  
      public IEnumerable&lt;ValidationResult&gt; Validate(ValidationContext validationContext)
      {
          if (string.IsNullOrWhiteSpace(ContactDetails.Email) &amp;&amp; string.IsNullOrWhiteSpace(ContactDetails.PhoneNumber))
          {
              yield return new ValidationResult("Either Email or Phone Number is required");
          }
      }
  }
  </code></pre>