MVC Controller Action should only handle certain parameter values, 404 otherwise
<p>I have an MVC controller action with one parameter that should only be called with certain values for that parameter (null/empty and some specific strings), in other cases it should not be hit (404 mostly). I've tried using a RegexRouteConstraint like below. But that doesn't filter the specific strings.</p>

<pre><code>var route = new Route("{soortAanbod}", new MvcRouteHandler())
{
    Defaults = new RouteValueDictionary(new { controller = "Homepage", action = "Index", soortAanbod = UrlParameter.Optional }),
    Constraints = new RouteValueDictionary(new { soortAanbod = new RegexRouteConstraint("a|b|c|d|e") }),
    DataTokens = new RouteValueDictionary { { "area", context.AreaName } }
};
context.Routes.Add("Homepage_soortAanbod", route);
</code></pre>

<p>The controller looks like this: <code>public ActionResult Index(string soortAanbod)</code></p>

<p>I've also tried using an action filter but that messes up other other filters. How can I make this route only match on the specified values for soortAanbod?</p>