This will handle Nancy content negotiation on errors so the client receives JSON, XML or HTML based on the client Accept headers

bootstrapper.cs

public class CustomBootstrapper : DefaultNancyBootstrapper
{
    protected override void ApplicationStartup(TinyIoCContainer container, IPipelines pipelines)
    {
        pipelines.OnError += (ctx, exception) =>
        {
            ctx.Items.Add("OnErrorException", exception);
            return null;
        };
    }
}


 

error.cs

 public class Error
{
    public string ErrorMessage { get; set; }
    public string FullException { get; set; }
}

errorstatuscodehandler.cs

public class ErrorStatusCodeHandler : IStatusCodeHandler
{
    private readonly IResponseNegotiator responseNegotiator;

    public ErrorStatusCodeHandler(IResponseNegotiator responseNegotiator)
    {
        this.responseNegotiator = responseNegotiator;
    }

    public bool HandlesStatusCode(HttpStatusCode statusCode, NancyContext context)
    {
        return statusCode == HttpStatusCode.InternalServerError;
    }

    public void Handle(HttpStatusCode statusCode, NancyContext context)
    {
        var response = new Negotiator(context);

        Error error = null;
        if (context.Items.ContainsKey("OnErrorException"))
        {
            var exception = context.Items["OnErrorException"] as Exception;
            error = new Error { ErrorMessage = exception.Message, FullException = exception.ToString() };
        }
        
        response.WithModel(new ErrorPageViewModel
        {
            Title = "Sorry, something went wrong",
            Summary = error == null ? "An unexpected error occurred." : error.ErrorMessage,
            Details = error == null ? null : error.FullException 
        })
        .WithStatusCode(statusCode)
        .WithView("Error");

        var errorresponse = responseNegotiator.NegotiateResponse(response, context);
        context.Response = errorresponse;
        
    }
}

Thanks, this is really great but it doesn't seem to work with Hosting.AspNet - any ideas?

@JeanTerblanche It may not be the same issue as you are seeing but I needed to call errorresponse.PreExecute(context); to prevent a null reference exception within Nancy.Responses.MaterialisingResponse.

L35 of errorstatuscodehandler.cs becomes

var errorresponse = responseNegotiator.NegotiateResponse(response, context);
errorresponse.PreExecute(context);
context.Response = errorresponse;

@MacsDickinson thank you very much - I was encountering the same issue and this fixed it.