title: ASP.Net Core MVC - Client-side validation for custom attribute
body: <p>In previous versions of the MVC framework custom validation would be achieved through implementing <code>IClientValidatable</code> and the <code>GetClientValidationRules</code> method.</p>
  
  <p>However in ASP.Net Core MVC <a href="https://github.com/aspnet/Mvc/search?utf8=%E2%9C%93&amp;q=IClientValidatable" rel="noreferrer">we do not have this interface</a>, although we do have <code>IClientModelValidator</code> which a defining a very similar method.  The implementation of which never gets called however.</p>
  
  <p>So - how do we implement client-side validation for a custom attribute in ASP.NET Core MVC?</p>