title: How to unit test ActionFilterAttribute
body: <p>I'm looking to test an ActionFilterAttribute in a .NET Core 2.0 API project and wondering the best way to go about it. Note, I'm not trying to test this through a controller action, merely test the ActionFilterAttribute itself.</p>
  
  <p>How might I go about testing this:</p>
  
  <pre><code>    public class ValidateModelAttribute : ActionFilterAttribute
      {
          public override void OnActionExecuting(ActionExecutingContext context)
          {
              if (!context.ModelState.IsValid)
              {
                  context.Result = new BadRequestObjectResult(context.ModelState);
              }
          }
      }
  </code></pre>