Nancy OnError will not accept a Response object? - json

Question

The Nancy documentation seems to say that Pipelines.OnError should return null - as opposed to BeforeResponse which allows both null and a Response object.
All the examples like this one and many code samples here on StackOverflow show a Response being returned in the OnError, just like in the BeforeRequest.
When I attempt to return an HTTPStatus string for the Pipelines.OnError, everything works OK!
But when I attempt to return a Response, I get a compiler error:
Operator '+=' cannot be applied to operands of type 'Nancy.ErrorPipeline' and 'lambda expression'
I'm emulating almost exactly the code in the Nancy example, except for the fact that mine is a TinyIocContainer while the example's is using a StructureMap container and a StructureMap derived bootstrapper
Here's my code:
const string errKey = "My proj error";
const string creationProblem = "Message creation (HTTP-POST)";
const string retrievalProblem = "Message retrieval (HTTP-GET)";
public void Initialize(IPipelines pipelines)
{
string jsonContentType = "application/json";
byte[] jsonFailedCreate = toJsonByteArray(creationProblem);
byte[] jsonFailedRetrieve = toJsonByteArray(retrievalProblem);
Response responseFailedCreate = new Response
{
StatusCode = HttpStatusCode.NotModified,
ContentType = jsonContentType,
Contents = (stream) =>
stream.Write(jsonFailedCreate, 0, jsonFailedCreate.Length)
};
Response responseFailedRetrieve = new Response
{
StatusCode = HttpStatusCode.NotFound,
ContentType = jsonContentType,
Contents = (stream) =>
stream.Write(jsonFailedRetrieve, 0, jsonFailedRetrieve.Length)
};
// POST - error in Create call
pipelines.OnError += (context, exception) =>
{
// POST - error during Create call
if (context.Request.Method == "POST")
return responsefailedCreate;
// GET - error during Retrieve call
else if (context.Request.Method == "GET")
return responseFailedRetrieve;
// All other cases - not supported
else
return HttpStatusCode.InternalServerError;
};
}
private byte[] toJsonByteArray(string plainString)
{
string jsonString = new JObject { { errKey, plainString } }.ToString();
byte[] result = Encoding.UTF8.GetBytes(jsonString);
return result;
}

Answer 1

I had the same problem and I found a nice approach to the problem: http://paulstovell.com/blog/consistent-error-handling-with-nancy.
you should override RequestStartup on the Bootstrapper, here my test code:
protected override void RequestStartup(TinyIoCContainer container, IPipelines pipelines, NancyContext context)
{
pipelines.OnError.AddItemToEndOfPipeline((ctx, ex) =>
{
DefaultJsonSerializer serializer = new DefaultJsonSerializer();
Response error = new JsonResponse(ex.Message,serializer);
error.StatusCode = HttpStatusCode.InternalServerError;
return error;
});
base.RequestStartup(container, pipelines, context);
}