How do I define the response to a missing claim in NancyFX?

Question

I have a module meant to enable administrators to manage users. Of course, this module requires not only authentication but also a specific claim. I have discovered that, if you are missing the claim in question, you actually get only a blank page as a response to your request. This isn't ideal.

How can I change that?

Module code below (if anyone needs to look)...

public class UserModule : NancyModule
{
    public UserModule()
        : base("/users")
    {
        this.RequiresAnyClaim(new[] { "evil-dictator" });

        Get["/"] = _ =>
        {
            ViewBag.UserName = Context.CurrentUser.UserName;
            return Negotiate.WithView("Index");
        };

        // Generate an invitation for a pre-approved user
        Get["/invite"] = _ =>
        {
            throw new NotImplementedException();
        };
    }
}

Answer 1

You can use an After hook to alter the response in case the claim is missing. Note that the response you get when you do not have the required claim has HTTP status code 403 Forbidden. Check for that in the After hook and alter the response as needed.

E.g. the following will redirect to the root - "/" - of the application, when the user does have the evil dictator claim:

public class UserModule : NancyModule
{
    public UserModule()
        : base("/users")
    {
        After += context =>
        {
            if (ctx.Response.StatusCode == HttpStatusCode.Forbidden)
              ctx.Response = this.Response.AsRedirect("/");
        }

        this.RequiresAnyClaim(new[] { "evil-dictator" });

        Get["/"] = _ =>
        {
            ViewBag.UserName = Context.CurrentUser.UserName;
            return Negotiate.WithView("Index");
        };

        // Generate an invitation for a pre-approved user
       Get["/invite"] = _ =>
       {
            throw new NotImplementedException();
       };
    }
}