title: RequiredIf Conditional Validation Attribute
body: <p>I was looking for some advice on the best way to go about implementing a validation attribute that does the following.</p>
  
  <p>Model</p>
  
  <pre><code>public class MyInputModel 
  {
      [Required]
      public int Id {get;set;}
  
      public string MyProperty1 {get;set;}
      public string MyProperty2 {get;set;}
      public bool MyProperty3 {get;set;}
  
  }
  </code></pre>
  
  <p>I want to have atleast prop1 prop2 prop3 with a value and if prop3 is the only value filled it it should not equal false.
  How would i go about writing a validation attribute(s?) for this?</p>
  
  <p>Thanks for any help!</p>