title: Why can't I use resources as ErrorMessage with DataAnnotations?
body: <p>Why can't I do like this?</p>
  
  <pre><code>[Required(ErrorMessage = "*")]
  [RegularExpression("^[a-zA-Z0-9_]*$", ErrorMessage = Resources.RegistrationModel.UsernameError)]
  public string Username { get; set; }
  </code></pre>
  
  <p>What is the error message telling me?</p>
  
  <blockquote>
    <p>An attribute argument must be a
    constant expression , typeof
    expression or array creation
    expression of an attribute parameter
    type.</p>
  </blockquote>