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ASP.NET Core, handle expected exceptions

I'm working on a pretty simple CRUD controller in ASP.NET core and now I'm facing the issue, that I need to handle errors like working with an not existing entity (returning a 404) or creating a duplicate (returning a 400).
So now I'm wondering what is the most idiomatic way in ASP.NET Core to fetch expected errors and return the correct status code for them.
One way could be to throw exceptions in the services that are responsible for the actual CRUD and catch them inside of the controller:
/// Inside service...
public void Create(Entity entityDetails) {
if (entityAlreadyExists(entityDetails)) {
throw new EntityDuplicateException();
}
// ...
}
/// Inside controller...
[HttpPost("{operatorClientId}")]
public void CreateEntity(Entity entityDetails) {
try {
_entityService.CreateEntity(entityDetails);
return Ok();
} catch (EntityDuplicateException e) { // Some self defined exception type
return BadRequest(/* Some details about the entity */);
}
}
This seems a bit repetitive to me because a lot of code would be always the same, but it makes clear what to expect from the API at the first glance. I think this could also get a bit messy when more exceptions need to be handled.
Another approach could be to implement a method for UseExceptionHandler, where all exception types are handled like:
switch (exceptionHandlerPathFeature.Error)
{
case EntityDuplicateException e:
context.Response.StatusCode = 400;
// More details to response here
break;
case EntityNotFoundException e:
context.Response.StatusCode = 404;
// More details to response here
break;
case {} e:
context.Response.StatusCode = 500;
// More details to response here
break;
}
This would be a single place for the handling of expected (and unexpected) exceptions. But you cannot see at first glance what the controller will return. And it can get harder to obtain all required information for a meaningful error message in the response (in the controller every information is directly available, with the ExceptionHandler I need to wrap all information into the exception).

An exception, as its name applies, should only be used in exceptional circumstances, not for control flow. An entity not being found or already existing is not exceptional; it's a common case that you should be handling explicitly in your data-access code. On the other hand, a deadlock in your database is.
Instead of having your data-access methods throw exceptions, make them return something like a Tuple of (bool success, string errorMessage). In the controller, check the success member - if it's false, return the errorMessage to the caller.
As for your global exception handler, leave it generic. It should do little more than log all exceptions to a central location, so that you can inspect those logs regularly to determine whether your app really is encountering exceptional circumstances on a regular basis - in which case you need to fix it.

Exception handling in .NET Core API consuming WCF Service

Im creating API in .NET Core, which consumes WCF Service. Accessing WCF service is realised, by calling any method, getting an exception (Access denied), and than calling LogIn method using cookie returned in header with first call response. Than, after login i want to retry my original call. All exceptions are the same, and only message string is different. Here is my code for one method call:
public async Task<List<scheduleElement>> getSchedule(DateTime start, DateTime end)
{
bool secondTry = false;
while (true)
{
try
{
var data = await _scheduleServiceClient.getScheduleAsync(start, end);
if (data.#return == null) return new List<scheduleElement>();
return data.#return.ToList();
}
catch (Exception e)
{
if (!secondTry && e.Message.StartsWith("Access denied for WebService method:"))
{
var logged = await LogIntoSOAPServices();
if (!logged) throw;
}
else throw;
secondTry = true;
}
}
}
Im using proxies generated with WCF Web Service Reference Provider
This works, but Im looking for a way to globaly handle exceptions and retry logic like this, because im going to have to copy and paste tons of code. I have Exception handler in my API but if i catch this exceptions with it im not able to retry method i originaly called.

A common library for cases like these is Polly;
https://github.com/App-vNext/Polly
Its part of the dotnet foundation i believe and is quite commonly used.
You can handle specific exceptions or results and act on that, e.g.
// Retry once
Policy
.Handle<SomeExceptionType>()
.Retry()
The logic can get quite complex. For webApi's i usually follow this guide from msdn:
https://learn.microsoft.com/en-us/dotnet/architecture/microservices/implement-resilient-applications/implement-http-call-retries-exponential-backoff-polly

UseExceptionHandler vs UseStatusCodePagesWithRedirects

I've been reading about errors handling in ASP.NET Core and I came across these 2 ways:
UseExceptionHandler("/error")
UseStatusCodePagesWithRedirects("/error/{0}");
I'm wondering what's the difference between the two? Both redirect to an error page so why use one over the other? I even saw some people using them both at the same time.

You are right that both middlewares do provide error pages. However, they have two different use cases which will make it useful to actually use both at the same time in an application. To understand the differences, let’s take a look at how the middlewares actually work internally.
This is essentially what the StatusCodePages middleware does:
// …
await _next(context);
// …
// Do nothing if a response body has already been provided.
if (context.Response.HasStarted
|| context.Response.StatusCode < 400
|| context.Response.StatusCode >= 600
|| context.Response.ContentLength.HasValue
|| !string.IsNullOrEmpty(context.Response.ContentType))
{
return;
}
var statusCodeContext = new StatusCodeContext(context, _options, _next);
await _options.HandleAsync(statusCodeContext);
It executes the pipeline by calling _next and after the call has returned (meaning that all following middlewares have executed), it will inspect the current response: Basically, if there’s an error status code or no content at all, it will execute the status code page, signalizing the HTTP status code.
The ExceptionHandler middleware on the other hand does something very different:
try
{
await _next(context);
}
catch (Exception ex)
{
// …
try
{
// …
await _options.ExceptionHandler(context);
// …
return;
}
catch (Exception ex2)
{
// Suppress secondary exceptions, re-throw the original.
_logger.ErrorHandlerException(ex2);
}
throw; // Re-throw the original if we couldn't handle it
}
This will try to invoke the middleware pipeline and catch any exception it might produce. Then, it will attempt to run the registered exception handler (which, when setting a path basically means to invoke that path internally and return its response).
So to sum this up:
The StatusCodePages middleware will handle non-successful status code responses and allows you to specify e.g. custom error pages for things like a 404 Not Found.
The ExceptionHandler middleware on the other hand will catch unhandled exceptions in your application and allows you to handle those gracefully for the end user.
Both middlewares have different purposes and actually don’t overlap in what they do. So it often makes sense to include both of them, unless you handle these issues differently of course; e.g. an API will probably not need status code pages, but might still want an exception handler that returns a generic failure and logs everything properly.

Exceptions and testing - how to unit test something for specific exception when I catch them all?

I'm writing MVC4 web application. Generally I try to put "try{}catch{}" block inside every controller method that returns ActionResult to the user. I do it in order to catch all Exceptions and display appropriate message, so user will never see something like:
"Reference not set to an instance of an object"
My controllers usually looks like this:
try
{
}
catch(MyFirstCustomException ex)
{
//set some message for the user and do some cleaning etc.
return ActionResult();
}
catch(MySecondCustomException ex) (and so on...)
{
//set some message for the user and do some cleaning etc.
return ActionResult();
}
catch(Exception ex)
{
//set some message for the user and do some cleaning etc.
return ActionResult();
}
However now I got the following situation: I have AccountController and a LogIn method, I want to write a unit test (using Microsoft Unit Testing Framework), that will assert that user which haven't activated his account, won't be able to log in. I have a special Exception named UserNotActivatedException that is thrown, when such attempt is detected. Problem is - since I catch all my exceptions within a controller, my test will never actually see this exception itself - thus the test will always fail. I managed to bypass the problem by creating special status enum for my model which looks like this:
public enum LoginViewModelStatus
{
NotLoggedIn = 0,
LoginSuccessfull = 1,
LoginFailed = 2,
UserNotActivatedException = 3,
UnknownErrorException = 100
}
and by setting it to a certain value when something is happening (so when I catch my special UserNotActivatedException - I set loginModelStatus to UserNotActivatedException and so on)
My questions:
Are there any nicer alternatives to this?
I'm thinking of using this design in other controllers as well, are there any downfalls here?
Is it good design to use a lot of custom exceptions for displaying messages for users, or would it be better to use more mini if(someCondition){return false;} tests?

You could wrap the code inside the try part in order to be able to unit test this part.
Here, the unit testable part is simply "wrapped" inside the MyUnitTestableMethod method :
try
{
MyUnitTestableMethod();
}
catch(MyFirstCustomException ex)
{
// ...
}
catch(MySecondCustomException ex) (and so on...)
{
// ...
}
catch(Exception ex)
{
// ...
}
KISS : Keep It Sanely Simple (or Keep It Simple and Stupid) :)

You should test that code returns expected results in all cases and more or less ignore how method does its work.
I.e. in your case Controller converts multiple exceptions into different view - test that when you feed data that causes exception scenario the Controller returns view you expect.
If lower levels of methods used by controller may throw exception - test them too, but this time for throwing particular exceptions.
It is up to you how many exceptions is enough. Good logging of exceptions is probably more important than variety. In most cases you should not show information from exception to a user anyway, but rather something like "Catastrophic error. If need assistance the error was logged with id AB455". All "expected exception" cases should be handled and presented to user as normal flow.
Note that it is ok to throw exceptions from actions as long as you have code that handles all exceptions. Action filter like HandleErrorAttribute can be used to configure exception policy for particular action/whole application.

It seems as you have your code "too stable". That is, your logic can never generate errors. It is good from a stability point of view but not very testable.
I would in this case have a class to handle the custom logic catch all exceptions generated from that class before returning ActionResult to separate the logic.
class ActionClass
{
public bool HandleLogin(...)
{
...
}
}
and use the class like this:
try
{
ActionClass action = new ActionClass();
action.HandleLogin(...)
}
// Catchblock here
This will allow you to test the logic.

What is the equivalent to SoapExtension for JSON WebMethods?

I've a few web methods that I use to call some external services like the Google Calendar API, obviously these can be extremely brittle.
Unfortunately I now realise that any error thrown on these methods are not causing an exception to bubble up to Global.asax which is where errors are getting logged in this application.
I have seen suggestions to wrap the method in a try/catch, which is a stupid way of doing it as there are a variety of errors that ASP.Net will silently swallow still.
In trying to find a solution I've seen a lot of references to SoapExtension, which is exactly what I want to do but doesn't get fired as I'm returning Json. What I really want is a way to catch the error just like that.
Any pointers appreciated, I still can't understand how the ASP.Net team could have thought that silently swallowing errors like this was a bright idea.
So for example a method like this:
[WebMethod]
[ExceptionHandling] //can I write a handler like this to catch exceptions from JSON webservices?
static public void DeleteItem(string id)
{
var api = new GoogleCalendarAPI(User.InternalUser());
api.DeleteEvent(id);
return "success";
}

There is no equivalent to SoapExtension for JSON WebMethods and having custom errors turned on in your production site will result in a generic error message being returned to the client, no error is ever raised on the server. You cannot circumvent this.
If you inspect the code using something like ILSpy, there is no way to pass a method or class to page WebMethods like SoapExtension. The error is swallowed by ASP.Net as it invokes the web method, the only notification you will get is a HTTP 500 error sent to the client with a total generic error message.
In 4.0, WebMethods get called by this:
// System.Web.Script.Services.RestHandler
internal static void ExecuteWebServiceCall(HttpContext context, WebServiceMethodData methodData)
{
try
{
//snip irrelevant code
RestHandler.InvokeMethod(context, methodData, rawParams);
}
catch (Exception ex)
{
RestHandler.WriteExceptionJsonString(context, ex);
}
}
So if invoking your method throws an error it will call the following code with a statusCode of 500, there's no re-throw in there and nothing else you can pass in called so unless I'm being blind it just gets swallowed silently. Even worse if you've got custom errors turned on, which any sane person will, it'll completely obfuscate the original cause:
// System.Web.Script.Services.RestHandler
internal static void WriteExceptionJsonString(HttpContext context, Exception ex, int statusCode)
{
//snip code setting up response
context.Response.TrySkipIisCustomErrors = true;
using (StreamWriter streamWriter = new StreamWriter(context.Response.OutputStream, new UTF8Encoding(false)))
{
if (ex is TargetInvocationException)
{
ex = ex.InnerException;
}
if (context.IsCustomErrorEnabled)
{
streamWriter.Write(JavaScriptSerializer.SerializeInternal(RestHandler.BuildWebServiceError(AtlasWeb.WebService_Error, string.Empty, string.Empty)));
}
else
{
streamWriter.Write(JavaScriptSerializer.SerializeInternal(RestHandler.BuildWebServiceError(ex.Message, ex.StackTrace, ex.GetType().FullName)));
}
streamWriter.Flush();
}
}
I can't see a way around it, looks like WebMethod is not ready for production code, shame.

It's not so much they get disappeared, it's more that they get passed out to the calling client. Since however you don't always want to (or should) reveal such intimate details of your service, you can prevent errors bubbling out of your service. This gives the impression of them disappearing.
Wrapping the inner detail in a try-catch is about the best way to cope with any errors. Within the method you're dealing with standard error trapping. So I think you'd want something like:
[WebMethod]
static public string DeleteItem(string id)
{
try
{
var api = new GoogleCalendarAPI(User.InternalUser());
api.DeleteEvent(id);
return "success";
}
catch(Exception ex)
{
log.fatal(ex);
return "error";
}
}
If anything throws an exception within the try-catch it'll be caught. ASP.Net won't interfere with it, unless the methods you are calling have been specifically coded to do so.
Edit
If the GoogleCalendarAPI class is in turn calling a method, such as ExecuteWebServiceCall with catches the Exception, then you'd have to parse the response. I'd hope they gave you some other clue, like a response code, to indicate an error state. You could then wrap that in an Exception, throw it have it caught by your default error handler.