Throw HttpResponseException or return Request CreateErrorResponse

Question

After reviewing an article Exception Handling in ASP NET Web API I am a bit confused as to when to throw an exception vs return an error response  I am also left wondering whether it is possible to modify the response when your method returns a domain specific model instead of HttpResponseMessage     So  to recap here are my questions followed by some code with case  s   Questions  Questions regarding Case  1   Should I always use HttpResponseMessage instead of a concrete domain model  so that the message can be customized  Can the message be customized if you are returning concrete domain model         Questions regarding Case  2 3 4   Should I be throwing an exception or returning error response  If the answer is  it depends   can you give situations examples on when to use one vs the other   What is the difference between throwing HttpResponseException vs Request CreateErrorResponse   The output to client seems identical    Should I always use HttpError to  wrap  response messages in errors  whether the exception is thrown or error response returned     Case Samples     CASE  1 public Customer Get string id        var customer    customerService GetById id       if  customer    null                var notFoundResponse   new HttpResponseMessage HttpStatusCode NotFound           throw new HttpResponseException notFoundResponse               var response   Request CreateResponse HttpStatusCode OK  customer         response Content Headers Expires   new DateTimeOffset DateTime Now AddSeconds 300        return customer                CASE  2 public HttpResponseMessage Get string id        var customer    customerService GetById id       if  customer    null                var notFoundResponse   new HttpResponseMessage HttpStatusCode NotFound           throw new HttpResponseException notFoundResponse             var response   Request CreateResponse HttpStatusCode OK  customer       response Content Headers Expires   new DateTimeOffset DateTime Now AddSeconds 300        return response        CASE  3 public HttpResponseMessage Get string id        var customer    customerService GetById id       if  customer    null                var message   String Format  customer with id   0  was not found   id           var errorResponse   Request CreateErrorResponse HttpStatusCode NotFound  message           throw new HttpResponseException errorResponse             var response   Request CreateResponse HttpStatusCode OK  customer       response Content Headers Expires   new DateTimeOffset DateTime Now AddSeconds 300        return response        CASE  4 public HttpResponseMessage Get string id        var customer    customerService GetById id       if  customer    null                var message   String Format  customer with id   0  was not found   id           var httpError   new HttpError message           return Request CreateErrorResponse HttpStatusCode NotFound  httpError             var response   Request CreateResponse HttpStatusCode OK  customer       response Content Headers Expires   new DateTimeOffset DateTime Now AddSeconds 300        return response      Update  To help further demonstrate cases  2 3 4 the following code snippet highlights several options that  can happen  when a customer is not found     if  customer    null           which of these 4 options is the best strategy for Web API          option 1  throw      var notFoundMessage   new HttpResponseMessage HttpStatusCode NotFound       throw new HttpResponseException notFoundMessage           option 2  throw w  HttpError      var message   String Format  Customer with id   0  was not found   id       var httpError   new HttpError message       var errorResponse   Request CreateErrorResponse HttpStatusCode NotFound  httpError       throw new HttpResponseException errorResponse           option 3  return      var message   String Format  Customer with id   0  was not found   id       return Request CreateErrorResponse HttpStatusCode NotFound  message          option 4  return w  HttpError      var message   String Format  Customer with id   0  was not found   id       var httpError   new HttpError message       return Request CreateErrorResponse HttpStatusCode NotFound  httpError

User · Answer

In error situations  I wanted to return a specific error details class  in whatever format the client requested instead of the happy path object   I want to have my controller methods return the domain specific happy path object and to throw an exception otherwise   The problem I had was that the HttpResponseException constructors do not allow domain objects   This is what I eventually came up with  public ProviderCollection GetProviders string providerName       try            return  providerPresenter GetProviders providerName           catch  BadInputValidationException badInputValidationException            throw new HttpResponseException Request CreateResponse HttpStatusCode BadRequest                                            badInputValidationException Result             Result is a class that contains error details  while ProviderCollection is my happy path result

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The approach I have taken is to just throw exceptions from the api controller actions and have an exception filter registered that processes the exception and sets an appropriate response on the action execution context   The filter exposes a fluent interface that provides a means of registering handlers for specific types of exceptions prior to registering the filter with global configuration   The use of this filter enables centralized exception handling instead of spreading it across the controller actions  There are however cases where I will catch exceptions within the controller action and return a specific response if it does not make sense to centralize the handling of that particular exception   Example registration of filter   GlobalConfiguration Configuration Filters Add      new UnhandledExceptionFilterAttribute        Register lt KeyNotFoundException gt  HttpStatusCode NotFound        Register lt SecurityException gt  HttpStatusCode Forbidden        Register lt SqlException gt            exception  request    gt                        var sqlException   exception as SqlException               if  sqlException Number  gt  50000                                var response              request CreateResponse HttpStatusCode BadRequest                   response ReasonPhrase     sqlException Message Replace Environment NewLine  String Empty                    return response                            else                               return request CreateResponse HttpStatusCode InternalServerError                                      UnhandledExceptionFilterAttribute class   using System  using System Collections Concurrent  using System Net  using System Net Http  using System Text  using System Web Http Filters   namespace Sample            lt summary gt          Represents the an attribute that provides a filter for unhandled exceptions           lt  summary gt      public class UnhandledExceptionFilterAttribute   ExceptionFilterAttribute                region UnhandledExceptionFilterAttribute                lt summary gt              Initializes a new instance of the  lt see cref  UnhandledExceptionFilterAttribute   gt  class               lt  summary gt          public UnhandledExceptionFilterAttribute     base                                 endregion           region DefaultHandler              lt summary gt              Gets a delegate method that returns an  lt see cref  HttpResponseMessage   gt               that describes the supplied exception               lt  summary gt               lt value gt              A  lt see cref  Func Exception  HttpRequestMessage  HttpResponseMessage    gt  delegate method that returns              an  lt see cref  HttpResponseMessage   gt  that describes the supplied exception               lt  value gt          private static Func lt Exception  HttpRequestMessage  HttpResponseMessage gt  DefaultHandler    exception  request    gt                        if exception    null                                return null                             var response              request CreateResponse lt string gt                   HttpStatusCode InternalServerError  GetContentOf exception                             response ReasonPhrase     exception Message Replace Environment NewLine  String Empty                return response                      endregion           region GetContentOf              lt summary gt              Gets a delegate method that extracts information from the specified exception               lt  summary gt               lt value gt              A  lt see cref  Func Exception  String    gt  delegate method that extracts information              from the specified exception               lt  value gt          private static Func lt Exception  string gt  GetContentOf    exception    gt                        if  exception    null                                return String Empty                             var result    new StringBuilder                 result AppendLine exception Message               result AppendLine                 Exception innerException   exception InnerException              while  innerException    null                                result AppendLine innerException Message                   result AppendLine                    innerException   innerException InnerException                              if DEBUG             result AppendLine exception StackTrace                endif              return result ToString                        endregion           region Handlers              lt summary gt              Gets the exception handlers registered with this filter               lt  summary gt               lt value gt              A  lt see cref  ConcurrentDictionary Type  Tuple    gt  collection that contains              the exception handlers registered with this filter               lt  value gt          protected ConcurrentDictionary lt Type  Tuple lt HttpStatusCode   Func lt Exception  HttpRequestMessage  HttpResponseMessage gt  gt  gt  Handlers                       get                               return  filterHandlers                                  private readonly ConcurrentDictionary lt Type  Tuple lt HttpStatusCode   Func lt Exception  HttpRequestMessage  HttpResponseMessage gt  gt  gt   filterHandlers   new ConcurrentDictionary lt Type  Tuple lt HttpStatusCode   Func lt Exception  HttpRequestMessage  HttpResponseMessage gt  gt  gt              endregion           region OnException HttpActionExecutedContext actionExecutedContext               lt summary gt              Raises the exception event               lt  summary gt               lt param name  actionExecutedContext  gt The context for the action  lt  param gt          public override void OnException HttpActionExecutedContext actionExecutedContext                        if actionExecutedContext    null    actionExecutedContext Exception    null                                return                             var type      actionExecutedContext Exception GetType                 Tuple lt HttpStatusCode   Func lt Exception  HttpRequestMessage  HttpResponseMessage gt  gt  registration   null               if  this Handlers TryGetValue type  out registration                                 var statusCode    registration Item1                  var handler       registration Item2                   var response      handler                      actionExecutedContext Exception GetBaseException                         actionExecutedContext Request                                        Use registered status code if available                 if  statusCode HasValue                                        response StatusCode   statusCode Value                                     actionExecutedContext Response    response                            else                                  If no exception handler registered for the exception type  fallback to default handler                 actionExecutedContext Response    DefaultHandler                      actionExecutedContext Exception GetBaseException    actionExecutedContext Request                                                     endregion           region Register lt TException gt  HttpStatusCode statusCode               lt summary gt              Registers an exception handler that returns the specified status code for exceptions of type  lt typeparamref name  TException   gt                lt  summary gt               lt typeparam name  TException  gt The type of exception to register a handler for  lt  typeparam gt               lt param name  statusCode  gt The HTTP status code to return for exceptions of type  lt typeparamref name  TException   gt   lt  param gt               lt returns gt              This  lt see cref  UnhandledExceptionFilterAttribute   gt  after the exception handler has been added               lt  returns gt          public UnhandledExceptionFilterAttribute Register lt TException gt  HttpStatusCode statusCode               where TException   Exception                        var type      typeof TException               var item      new Tuple lt HttpStatusCode   Func lt Exception  HttpRequestMessage  HttpResponseMessage gt  gt                   statusCode  DefaultHandler                             if   this Handlers TryAdd type  item                                 Tuple lt HttpStatusCode   Func lt Exception  HttpRequestMessage  HttpResponseMessage gt  gt  oldItem   null                   if  this Handlers TryRemove type  out oldItem                                         this Handlers TryAdd type  item                                                return this                     endregion           region Register lt TException gt  Func lt Exception  HttpRequestMessage  HttpResponseMessage gt  handler               lt summary gt              Registers the specified exception  lt paramref name  handler   gt  for exceptions of type  lt typeparamref name  TException   gt                lt  summary gt               lt typeparam name  TException  gt The type of exception to register the  lt paramref name  handler   gt  for  lt  typeparam gt               lt param name  handler  gt The exception handler responsible for exceptions of type  lt typeparamref name  TException   gt   lt  param gt               lt returns gt              This  lt see cref  UnhandledExceptionFilterAttribute   gt  after the exception  lt paramref name  handler   gt               has been added               lt  returns gt               lt exception cref  ArgumentNullException  gt The  lt paramref name  handler   gt  is  lt see langword  null   gt   lt  exception gt          public UnhandledExceptionFilterAttribute Register lt TException gt  Func lt Exception  HttpRequestMessage  HttpResponseMessage gt  handler               where TException   Exception                       if handler    null                              throw new ArgumentNullException  handler                               var type      typeof TException               var item      new Tuple lt HttpStatusCode   Func lt Exception  HttpRequestMessage  HttpResponseMessage gt  gt                   null  handler                             if   this Handlers TryAdd type  item                                 Tuple lt HttpStatusCode   Func lt Exception  HttpRequestMessage  HttpResponseMessage gt  gt  oldItem   null                   if  this Handlers TryRemove type  out oldItem                                         this Handlers TryAdd type  item                                                return this                     endregion           region Unregister lt TException gt                 lt summary gt              Unregisters the exception handler for exceptions of type  lt typeparamref name  TException   gt                lt  summary gt               lt typeparam name  TException  gt The type of exception to unregister handlers for  lt  typeparam gt               lt returns gt              This  lt see cref  UnhandledExceptionFilterAttribute   gt  after the exception handler              for exceptions of type  lt typeparamref name  TException   gt  has been removed               lt  returns gt          public UnhandledExceptionFilterAttribute Unregister lt TException gt                where TException   Exception                       Tuple lt HttpStatusCode   Func lt Exception  HttpRequestMessage  HttpResponseMessage gt  gt  item   null               this Handlers TryRemove typeof TException   out item                return this                     endregion           Source code can also be found here

User · Answer

Case  1   Not necessarily  there are other places in the pipeline to modify the response  action filters  message handlers    See above -- but if the action returns a domain model  then you can t modify the response inside the action    Cases  2-4   The main reasons to throw HttpResponseException are   if you are returning a domain model but need to handle error cases  to simplify your controller logic by treating errors as exceptions  These should be equivalent  HttpResponseException encapsulates an HttpResponseMessage  which is what gets returned back as the HTTP response   e g   case  2 could be rewritten as  public HttpResponseMessage Get string id        HttpResponseMessage response      var customer    customerService GetById id       if  customer    null                response   new HttpResponseMessage HttpStatusCode NotFound             else               response   Request CreateResponse HttpStatusCode OK  customer           response Content Headers Expires   new DateTimeOffset DateTime Now AddSeconds 300              return response          but if your controller logic is more complicated  throwing an exception might simplify the code flow  HttpError gives you a consistent format for the response body and can be serialized to JSON XML etc  but it s not required  e g   you may not want to include an entity-body in the response  or you might want some other format

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If you are not returning HttpResponseMessage and instead are returning entity model classes directly  an approach which I have found useful is to add the following utility function to my controller  private void ThrowResponseException HttpStatusCode statusCode  string message        var errorResponse   Request CreateErrorResponse statusCode  message       throw new HttpResponseException errorResponse       and simply call it with the appropriate status code and message

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I want to point out that it has been my experience that if throwing an HttpResponseException instead of returning an HttpResponseMessage in a webapi 2 method  that if a call is made immediately to IIS Express it will timeout or return a 200 but with a html error in the response   Easiest way to test this is to make   ajax call to a method that throws a HttpResponseException and in the errorCallBack in ajax make an immediate call to another method or even a simple http page  You will notice the imediate call will fail  If you add a break point or a settimeout   in the error call back to delay the second call a second or two giving the server time to recover it works correctly  This makes no since but its almost like the throw HttpResponseException causes the server side listener thread to exit and restart causing a split second of no server accepting connections or something    Update  The root cause of the wierd Ajax connection Timeout is if an ajax call is made quick enough the same tcp connection is used  I was raising a 401 error ether by returning a HttpResonseMessage or throwing a HTTPResponseException which was returned to the browser ajax call  But along with that call MS was returning a Object Not Found Error because in Startup Auth vb app UserCookieAuthentication was enabled so it was trying to return intercept the response and add a redirect but it errored with Object not Instance of an Object  This Error was html but was appended to the response after the fact so only if the ajax call was made quick enough and the same tcp connection used did it get returned to the browser and then it got appended to the front of the next call  For some reason Chrome just timedout  fiddler pucked becasue of the mix of json and htm but firefox rturned the real error   So wierd but packet sniffer or firefox was the only way to track this one down   Also it should be noted that if you are using Web API help to generate automatic help and you return HttpResponseMessage then you should add a   System Web Http Description ResponseType typeof CustomReturnedType       attribute to the method so the help generates correctly  Then   return Request CreateResponse lt CustomReturnedType gt  objCustomeReturnedType     or on error   return Request CreateErrorResponse  System Net HttpStatusCode InternalServerError  new Exception  An Error Ocurred       Hope this helps someone else who may be getting random timeout or server not available immediately after throwing a HttpResponseException   Also returning an HttpResponseException has the added benefit of not causing Visual Studio to break on an Un-handled exception usefull when the error being returned is the AuthToken needs to be refreshed in a single page app   Update   I am retracting my statement about IIS Express timing out  this happened to be a mistake in my client side ajax call back  it turns out since Ajax 1 8 returning   ajax   and returning   ajax    then   both return promise but not the same chained promise then   returns a new promise which caused the order of execution to be wrong  So when the then   promise completed it was a script timeout  Weird gotcha but not an IIS express issue a problem between the Keyboard and chair

User · Answer

Another case for when to use HttpResponseException instead of Response CreateResponse HttpStatusCode NotFound   or other error status code  is if you have transactions in action filters and you want the transactions to be rolled back when returning an error response to the client   Using Response CreateResponse will not roll the transaction back  whereas throwing an exception will

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Do not throw an HttpResponseException or return an HttpResponesMessage for errors - except if the intent is to end the request with that exact result   HttpResponseException s are not handled the same as other exceptions  They are not caught in Exception Filters  They are not caught in the Exception Handler  They are a sly way to slip in an HttpResponseMessage while terminating the current code s execution flow   Unless the code is infrastructure code relying on this special un-handling  avoid using the HttpResponseException type   HttpResponseMessage s are not exceptions  They do not terminate the current code s execution flow  They can not be filtered as exceptions  They can not be logged as exceptions  They represent a valid result - even a 500 response is  a valid non-exception response      Make life simpler   When there is an exceptional error case  go ahead and throw a normal  NET exception - or a customized application exception type  not deriving from HttpResponseException  with desired  http error response  properties such as a status code - as per normal exception handling   Use Exception Filters   Exception Handlers   Exception Loggers to do something appropriate with these exceptional cases  change add status codes  add tracking identifiers  include stack traces  log   By avoiding HttpResponseException the  exceptional case  handling is made uniform and can be handled as part of the exposed pipeline  For example one can turn a  NotFound  into a 404 and an  ArgumentException  into a 400 and a  NullReference  into a 500 easily and uniformly with application-level exceptions - while allowing extensibility to provide  basics  such as error logging

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I like Oppositional answer  Anyway  I needed a way to catch the inherited Exception and that solution doesn t satisfy all my needs   So I ended up changing how he handles OnException and this is my version  public override void OnException HttpActionExecutedContext actionExecutedContext       if  actionExecutedContext    null    actionExecutedContext Exception    null          return           var type   actionExecutedContext Exception GetType        Tuple lt HttpStatusCode   Func lt Exception  HttpRequestMessage  HttpResponseMessage gt  gt  registration   null      if   this Handlers TryGetValue type  out registration             tento di vedere se ho registrato qualche eccezione che eredita dal tipo di eccezione sollevata  in ordine di registrazione        foreach  var item in this Handlers Keys             if  type IsSubclassOf item                 registration   this Handlers item               break                                se ho trovato un tipo compatibile  uso la sua gestione    if  registration    null          var statusCode   registration Item1        var handler   registration Item2         var response   handler           actionExecutedContext Exception GetBaseException             actionExecutedContext Request                    Use registered status code if available       if  statusCode HasValue             response StatusCode   statusCode Value                 actionExecutedContext Response   response          else            If no exception handler registered for the exception type  fallback to default handler       actionExecutedContext Response   DefaultHandler actionExecutedContext Exception GetBaseException    actionExecutedContext Request                   The core is this loop where I check if the exception type is a subclass of a registered type   foreach  var item in this Handlers Keys        if  type IsSubclassOf item             registration   this Handlers item           break            my2cents

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As far as I can tell  whether you throw an exception  or you return Request CreateErrorResponse  the result is the same   If you look at the source code for System Web Http dll  you will see as much   Take a look at this general summary  and a very similar solution that I have made  Web Api  HttpError  and the behavior of exceptions

[c#] Throw HttpResponseException or return Request.CreateErrorResponse?

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