How to setup an exception handler in EmbedIO Web API

Question

I'm using EmbedIO with the Web API module.

I'd like to have an exception handler that will catch all the unhandled exceptions and return a suited HTTP error code according to the exception type. However, it's not clear if this can be achieved.

The class WebModuleBase exposes a property called OnUnhandledException that can be set to an ExceptionHandlerCallback, but when the callback is invoked, the response's status code has already been set to HttpStatusCode.InternalServerError, as stated in the code comments.

This is not convenient since I want to set the response code myself.

WebModuleBase exposes a different property called OnHttpException that can be set to a HttpExceptionHandlerCallback. This can be set to HttpExceptionHandler.DataResponse(ResponseSerializer.Json) which partially solves the issue.

The main concern now is that the application exceptions must be converted to HttpException in the controllers.

I'd like to throw custom exceptions from the domain code, get them in an exception handler and just return a HTTPException in there, according to the initial exception.

Basically something similar to Exception Filters in ASP.NET Web API.

Here's the code to setup the web server:

var webApiModule = new WebApiModule("/api", ResponseSerializer.Json)
    .WithController<MyController>();
webApiModule.OnUnhandledException = ExceptionHandler.DataResponseForException();
webApiModule.OnHttpException = ExceptionHandler.DataResponseForHttpException();

WebServerEmbedded = new EmbedIO.WebServer(
    opt => opt
        .WithUrlPrefix(url)
        .WithMode(HttpListenerMode.EmbedIO))
        .WithModule(null, webApiModule);

These are the delegates used for exception handlers:

internal static class ExceptionHandler
{
    public static ExceptionHandlerCallback DataResponseForException()
    {
        return (context, exception) => ResponseSerializer.Json(context, exception.Message);
    }

    public static HttpExceptionHandlerCallback DataResponseForHttpException()
    {
        return (context, httpException) => ResponseSerializer.Json(context, httpException.Message);
    }
}

Thanks.

Answer 1

Exceptions, as well as HTTP exceptions, are handled by EmbedIO at both module and server level (each nested module group introduces a further level, but that's beyond the point).

The catch clause for HTTP exceptions always comes before the "general-purpose" catch clause, for the obvious reason that HTTP exceptions are exceptions themselves and need to be sorted out. Therefore, if an exception handler throws a HTTP exception, the latter must be handled at an outer level.

In opther words, you can write a module-level exception handler that throws a HTTP exception, then use a server-level HTTP exception handler to generate the appropriate response.

var webApiModule = new WebApiModule("/api", ResponseSerializer.Json)
    .WithController<MyController>()
    .HandleUnhandledException(ExceptionHandler.DataResponseForException));

WebServerEmbedded = new EmbedIO.WebServer(
    opt => opt
        .WithUrlPrefix(url)
        .WithMode(HttpListenerMode.EmbedIO))
    .WithModule(webApiModule)
    .HandleHttpException(ExceptionHandler.DataResponseForHttpException);

internal static class ExceptionHandler
{
    public static Task DataResponseForException(IHttpContext context, Exception exception)
    {
        // Replace ANY_VALID_STATUS CODE with, well, any valid status code.
        // Of course you can use different status codes according to, for example,
        // the type of exception.
        throw new HttpException(ANY_VALID_STATUS_CODE, exception.Message);
    }

    public static Task DataResponseForHttpException(IHttpContext context, IHttpException httpException)
    {
        context.Response.StatusCode = (int)HttpStatusCode.OK;
        return ResponseSerializer.Json(context, httpException.Message);
    }
}

---

EDIT: There's an even simpler way, if you need it for custom exceptions: just have your exceptions implement IHttpException.

Here you can see how IHttpException methods are used by the HTTP exception handling code.

Here is an example of probably the most obscure method, PrepareResponse.

EDIT: I added setting the status code in DataResponseForHttpException.