title: C# ASP.NET MVC Foolproof RequiredIf - how to set ErrorMessage to be the value of other (string) property?
body: <p>That's what I'm looking for:</p>
  
  <pre><code>[RequiredIf("IsNewClient", true, ErrorMessage =*The value of other (string) property*)]
  </code></pre>
  
  <p>That's would be even better:</p>
  
  <pre><code>[RequiredIf("IsNewClient", true, ErrorMessage =*Calling a function and get the value*)]
  </code></pre>