Is there any way within middleware running on ASP.NET Core 2.2 to detect if the request is for an ApiController? - exception

Question

I have an application with both MVC and 'new' ApiController endpoints in ASP.NET Core 2.2 co-existing together.
Prior to adding the API endpoints, I have been using a global exception handler registered as middleware using app.UseExceptionHandler((x) => { ... } which would redirect to an error page.
Of course, that does not work for an API response and I would like to return an ObjectResult (negotiated) 500 result with a ProblemDetails formatted result.
The problem is, I'm not sure how to reliably determine in my 'UseExceptionHandler' lambda if I am dealing with an MVC or a API request. I could use some kind of request URL matching (eg. /api/... prefix) but I would like a more robust solution that won't come back to bite me in the future.
Rough psuedo-code version of what I'm trying to implement is:
app.UseExceptionHandler(x =>
{
x.Run(async context =>
{
// extract the exception that was thrown
var ex = context.Features.Get<IExceptionHandlerFeature>()?.Error;
try
{
// generically handle the exception regardless of what our response needs to look like by logging it
// NOTE: ExceptionHandlerMiddleware itself will log the exception
// TODO: need to find a way to see if we have run with negotiation turned on (in which case we are API not MVC!! see below extensions for clues?)
// TODO: ... could just use "/api/" prefix but that seems rubbish
if (true)
{
// return a 500 with object (in RFC 7807 form) negotiated to the right content type (eg. json)
}
else
{
// otherwise, we handle the response as a 500 error page redirect
}
}
catch (Exception exofex)
{
// NOTE: absolutely terrible if we get into here
log.Fatal($"Unhandled exception in global error handler!", exofex);
log.Fatal($"Handling exception: ", ex);
}
});
});
}
Any ideas?
Cheers!

Answer 1

This might be a bit different than what you expect, but you could just check if the request is an AJAX request.
You can use this extension:
public static class HttpRequestExtensions
{
public static bool IsAjaxRequest(this HttpRequest request)
{
if (request == null)
throw new ArgumentNullException(nameof(request));
if (request.Headers == null)
return false;
return request.Headers["X-Requested-With"] == "XMLHttpRequest";
}
}
And then middleware with an invoke method that looks like:
public async Task Invoke(HttpContext context)
{
if (context.Request.IsAjaxRequest())
{
try
{
await _next(context);
}
catch (Exception ex)
{
//Handle the exception
await HandleExceptionAsync(context, ex);
}
}
else
{
await _next(context);
}
}
private static Task HandleExceptionAsync(HttpContext context, Exception exception)
{
//you can do more complex logic here, but a basic example would be:
var result = JsonConvert.SerializeObject(new { error = "An unexpected error occurred." });
context.Response.ContentType = "application/json";
context.Response.StatusCode = 500;
return context.Response.WriteAsync(result);
}
see this SO answer for a more detailed version.

Answer 2

If you want to check whether the request is routed to ApiController, you could try IExceptionFilter to hanlde the exceptions.
public class CustomExceptionFilter : IExceptionFilter
{
public void OnException(ExceptionContext context)
{
if (IsApi(context))
{
HttpStatusCode status = HttpStatusCode.InternalServerError;
var message = context.Result;
//You can enable logging error
context.ExceptionHandled = true;
HttpResponse response = context.HttpContext.Response;
response.StatusCode = (int)status;
response.ContentType = "application/json";
context.Result = new ObjectResult(new { ErrorMsg = message });
}
else
{
}
}
private bool IsApi(ExceptionContext context)
{
var controllerActionDesc = context.ActionDescriptor as ControllerActionDescriptor;
var attribute = controllerActionDesc
.ControllerTypeInfo
.CustomAttributes
.FirstOrDefault(c => c.AttributeType == typeof(ApiControllerAttribute));
return attribute == null ? false : true;
}
}

Answer 3

Thanks to all of the advice from others, but I have realised after some more thought and ideas from here that my approach wasn't right in the first place - and that I should be handling most exceptions locally in the controller and responding from there.
I have basically kept my error handling middleware the same as if it was handling MVC unhandled exceptions. The client will get a 500 with a HTML response, but at that point there isn't much the client can do anyway so no harm.
Thanks for your help!