Question

Note that this is for an ApiController in MVC 4 although I think it shouldn’t change anything.

 public class OAuthFilter : System.Web.Http.ActionFilterAttribute
 {
       public override void OnActionExecuting(System.Web.Http.Controllers.HttpActionContext actionContext)
       {
              if (checkVerified())
              {
                   // How to raise a 401 or some other type of exception.
              }
       }        
 }

Answer 1

You can set the result property of the HttpActionContext:

public override void OnActionExecuting(HttpActionContext actionContext)
{
    if (checkVerified())
    {
        actionContext.Response = 
            new HttpResponseMessage(HttpStatusCode.Unauthorized);
    }
}

you may probably just throw:

throw new HttpResponseException(HttpStatusCode.Unauthorized);

but I haven’t checked that one.