Web API routing / Resource not found when I omit some parameters - asp.net-web-api-routing

Question

In WebApiConfig I have following:
config.Routes.MapHttpRoute("Alerts", "alerts/{username}/{scope}/{language}",new { controller = "Alerts", language = RouteParameter.Optional });
In my AlertsController I have this:
public async Task<HttpResponseMessage> GetAsync(string username, string scope, string language = "en")
{
if (string.IsNullOrWhiteSpace(username))
{
return this.Request.CreateErrorResponse(HttpStatusCode.BadRequest, "Username cannot be null.");
}
if (string.IsNullOrWhiteSpace(scope))
{
return this.Request.CreateErrorResponse(HttpStatusCode.BadRequest, "Scope cannot be null.");
}
var scp = this._scopeFactory.GetScope(scope);
var alerts = await scp.GetAlerts(username, scope).ConfigureAwait(false);
return this.Request.CreateResponse(HttpStatusCode.OK, alerts);
}
What I want to achieve, is when I type in my browser http://www.localhost.com/alerts (without giving a username and scope) to see "Username cannot be null" and "Scope cannot be null" in json format.
Right now I get Resource not found, which makes sense since the router can not match such a resource. Do you know if is possible to achieve what I want with some Configuration change (or maybe something else)?

Answer 1

I had to set nullable parameters to make this work.
public async Task<HttpResponseMessage> GetAsync(string username = null, string scope = null, string language = "en")
In WebApiConfig I had to make them optional as well:
config.Routes.MapHttpRoute("Alerts", "alerts/{username}/{scope}/{language}",new { controller = "Alerts", username = RouteParameter.Optional, scope = RouteParameter.Optional, language = RouteParameter.Optional });