Losing Exception information with HttpError

Question

I'm trying to get a custom errorid (hashed guid) to an HttpError when exceptions are raised through an application. So a message like "xxxxxx - error" is passed to the client.

We have a custom ExceptionFilterAttribute to catch and log exceptions:

public class ExceptionFilter : ExceptionFilterAttribute
{
    ...
    public override void OnException(HttpActionExecutedContext context)
    {
        Guid guid = Guid.NewGuid();
        context.Exception.Data.Add("guid", guid);
        _log.WriteEvent(message, context.Exception, guid);
    }
}

A custom MediaTypeFormatter to send this (among other types) as json back to the client:

public class DotNetJsonFormatter : MediaTypeFormatter
{
    ...
    public override Task WriteToStreamAsync(Type type, object value, Stream stream, HttpContent content, TransportContext transportContext)
    {
        return Task.Factory.StartNew(() =>
        {
            if (typeof(Exception).IsAssignableFrom(type)) {
                // write a json string to the stream containing the message and id
            } else if (typeof(HttpError).IsAssignableFrom(type)) {
                // write a json string to the stream containing the message and id
            }
        });
    }
}

My problem lies with HttpError - all it contains is a dictionary containing the message, type, stack trace etc - I thought setting the data might pull this through (I've checked the HttpError constructor and that's all that's pulled from the exception).

I can't re-write the application to throw custom exceptions so the id ends up in the message.
I 'could' using reflection, alter the exception message to contain the id in the filter but it feels hacky.
Thought about re-throwing a new exception with the id in the message from the filter but this seems to bypass the formatter altogether
Tried setting the Exception property of the filter to set the underlying exception but it has no effect.

Am I stuck with the reflection technique?

Unless I just bypass the formatter and set the response of the `HttpActionExecutedContext` directly? — Phil Cooper, Sep 24 '15 at 10:02
What is the type of `context.Exception`? If it's HttpError then you might have to use the items.add instead of data.add — Piyush, Sep 24 '15 at 10:14

score 0 · Answer 1 · answered Sep 24 '15 at 13:57

I hate answering my own questions but I will anyway.

So one option I didn't consider (too much inside box thinking) was setting the response in the filter rather than letting the formatter handle it.

The updated filter:

public class ExceptionFilter : ExceptionFilterAttribute
{
    public ExceptionFilter(ILogger log, IClientExceptionFormatter exceptionFormatter)
    {
        ...
        _log = log;
        _exceptionFormatter = exceptionFormatter;
    }

    public override void OnException(HttpActionExecutedContext context)
    {
        ...
        _log.Error(context.Exception);
        context.Response = new ExceptionJsonResponse(context.Exception, _exceptionFormatter);
    }
    ...
}

The ExceptionJsonResponse class:

public class ExceptionJsonResponse : HttpResponseMessage
{
    public ExceptionJsonResponse(Exception exception, IClientExceptionFormatter formatter)
    {
        ...
        _exception = exception;
        _formatter = formatter;

        Content = CreateContent();
        StatusCode = HttpStatusCode.InternalServerError;
    }

    private HttpContent CreateContent()
    {
        HttpContent content = new StringContent(_formatter.GetJson(_exception));
        content.Headers.ContentType = new MediaTypeHeaderValue(Common.ContentType.ApplicationJson);
        return content;
    }
    ...
}

This way bypasses the formatter probably because the response is a HttpResponseMessage. I can then alter the client message to my hearts content.

Losing Exception information with HttpError

1 Answers1