title: Data Annotations make a value Required when another model value is not empty?
body: <p>Is it possible to create a data annotation that allows me to say if QuestionType = "dropdown" then SelectedValue is Required?</p>
  
  <p>Here is my Model:</p>
  
  <pre><code>public class QuestionViewModel
  {
      public int? Id { get; set; }
  
      public string QuestionType { get; set; }
  
      public string SubType { get; set; }
  
      public string Text { get; set; }
  
      public int SortOrder { get; set; }
  
      public bool IsHidden { get; set; }
  
      public int SelectedValue { get; set; }
  
      public List&lt;QuestionOptionViewModel&gt; Options { get; set; }
  }
  </code></pre>
  
  <p>What I want to say is if the QuestionType is of a specific type (dropdown) then SelectedValue is Required.</p>