Impossible to show Custom Error page with Nancy on OWIN - owin

Question

I have a website using Nancy which is hosted using OWIN.
In my Startup.cs file I define the PassThroughOptions as follows:
public void Configuration(IAppBuilder app)
{
app.UseNancy(o => {
o.PassThroughWhenStatusCodesAre(
HttpStatusCode.NotFound,
HttpStatusCode.InternalServerError
);
o.Bootstrapper = new Bootstrapper();
});
app.UseStageMarker(PipelineStage.MapHandler);
}
I need to pass-through the NotFound requests, so that things like my bundled .less files or miniprofiler-results or static files in the root of my site (robots.txt or sitemap.xml) work.
I also have a custom StatusCodeHandler for the 404 code, which also checks a custom header to distinguish between static files (or .less bundles/miniprofiler) and actual stuff that is not found in my modules' methods.
public void Handle(HttpStatusCode statusCode, NancyContext context)
{
Log.Warn("Not found: " + context.Request.Url);
base.Handle(statusCode, context, "Errors/NotFound");
}
This handler then should actually show the error page.
protected void Handle(HttpStatusCode statusCode, NancyContext context, string view)
{
var response = new Negotiator(context)
.WithModel(GetErrorModel(context))
.WithStatusCode(statusCode)
.WithView(view);
context.Response = responseNegotiator.NegotiateResponse(response, context);
}
But the error page is never shown. The request is processed three times and eventually the default IIS error page is shown (using errorMode="Custom" for httpErrors) or simply a white page (using existingResponse="PassThrough" for httpErrors).
Is there any way to display something so simple as a custom error page when hosting a Nancy website on OWIN?

Answer 1

What you've got there looks good, it looks like you've be using the Hosting Nancy with Owin docs.
Here's what works for me:
The Startup.cs (required for Owin): (We've both coded the configuration function differently, you're just using the extension helper while I'm not. Same result. This is in my App.Web project.)
public class Startup
{
public void Configuration(IAppBuilder app)
{
app.UseNancy(options =>
{
options.Bootstrapper = new BootStrapper();
options.PerformPassThrough = context => context.Response.StatusCode == HttpStatusCode.NotFound;
});
app.UseStageMarker(PipelineStage.MapHandler);
}
}
404 handler: (As per the docs, doesn't matter where this is in the project, by implementing IStatusCodeHandler it'll be automatically picked up by Nancy This is in my App.WebApi project with other module classes.)
public class StatusCode404Handler : IStatusCodeHandler
{
public bool HandlesStatusCode(HttpStatusCode statusCode, NancyContext context)
{
return statusCode == HttpStatusCode.NotFound;
}
public void Handle(HttpStatusCode statusCode, NancyContext context)
{
var response = new GenericFileResponse("statuspages/404.html", "text/html")
{
StatusCode = statusCode
};
context.Response = response;
}
}
The 'statuspages' folder in my App.Web project:
Check this SO post for a comparison of using GenericFileReponse or ViewRenderer (How to display my 404 page in Nancy?).