ASP NET Core Web API exception handling

Question

I am using ASP NET Core for my new REST API project after using regular ASP NET Web API for many years  I don t see any good way to handle exceptions in ASP NET Core Web API  I tried to implement exception handling filter attribute   public class ErrorHandlingFilter   ExceptionFilterAttribute       public override void OnException ExceptionContext context                HandleExceptionAsync context           context ExceptionHandled   true             private static void HandleExceptionAsync ExceptionContext context                var exception   context Exception           if  exception is MyNotFoundException              SetExceptionResult context  exception  HttpStatusCode NotFound           else if  exception is MyUnauthorizedException              SetExceptionResult context  exception  HttpStatusCode Unauthorized           else if  exception is MyException              SetExceptionResult context  exception  HttpStatusCode BadRequest           else             SetExceptionResult context  exception  HttpStatusCode InternalServerError              private static void SetExceptionResult          ExceptionContext context           Exception exception           HttpStatusCode code                context Result   new JsonResult new ApiResponse exception                         StatusCode    int code                      And here is my Startup filter registration   services AddMvc options   gt        options Filters Add new AuthorizationFilter         options Filters Add new ErrorHandlingFilter           The issue I was having is that when exception occurres in my AuthorizationFilter it s not being handled by ErrorHandlingFilter  I was expecting it to be caught there just like it worked with old ASP NET Web API    So how can I catch all application exceptions as well as any exceptions from Action Filters

User · Answer

First  configure ASP NET Core 2 Startup to re-execute to an error page for any errors from the web server and any unhandled exceptions   public void Configure IApplicationBuilder app  IHostingEnvironment env        if  env IsDevelopment                 Debug config here          else           app UseStatusCodePagesWithReExecute   Error            app UseExceptionHandler   Error                 More config        Next  define an exception type that will let you throw errors with HTTP status codes   public class HttpException   Exception       public HttpException HttpStatusCode statusCode    StatusCode   statusCode        public HttpStatusCode StatusCode   get  private set        Finally  in your controller for the error page  customize the response based on the reason for the error and whether the response will be seen directly by an end user  This code assumes all API URLs start with  api     AllowAnonymous  public IActionResult Error            Gets the status code from the exception or web server      var statusCode   HttpContext Features Get lt IExceptionHandlerFeature gt     Error is HttpException httpEx           httpEx StatusCode    HttpStatusCode Response StatusCode          For API errors  responds with just the status code  no page       if  HttpContext Features Get lt IHttpRequestFeature gt    RawTarget StartsWith   api    StringComparison Ordinal           return StatusCode  int statusCode           Creates a view model for a user-friendly error page      string text   null      switch  statusCode            case HttpStatusCode NotFound  text    Page not found    break             Add more as desired            return View  Error   new ErrorViewModel   RequestId   Activity Current  Id    HttpContext TraceIdentifier  ErrorText   text         ASP NET Core will log the error detail for you to debug with  so a status code may be all you want to provide to a  potentially untrusted  requester  If you want to show more info  you can enhance HttpException to provide it  For API errors  you can put JSON-encoded error info in the message body by replacing return StatusCode    with return Json

User · Answer

To Configure exception handling behavior per exception type you can use Middleware from NuGet packages    Community AspNetCore ExceptionHandling NewtonsoftJson  for ASP NET Core 2 0  Community AspNetCore ExceptionHandling Mvc for ASP NET Core 2 1      Code sample   public void ConfigureServices IServiceCollection services        services AddMvc         services AddExceptionHandlingPolicies options   gt                options For lt InitializationException gt    Rethrow             options For lt SomeTransientException gt    Retry ro   gt  ro MaxRetryCount   2  NextPolicy             options For lt SomeBadRequestException gt             Response e   gt  400               Headers  h  e    gt  h  X-MyCustomHeader     e Message               WithBody  req sw  exception    gt                                        byte   array   Encoding UTF8 GetBytes exception ToString                         return sw WriteAsync array  0  array Length                               NextPolicy                Ensure that all exception types are handled by adding handler for generic exception at the end          options For lt Exception gt             Log lo   gt                                lo EventIdFactory    c  e    gt  new EventId 123   UnhandlerException                    lo Category    context  exception    gt   MyCategory                           Response null  ResponseAlreadyStartedBehaviour GoToNextHandler               ClearCacheHeaders                WithObjectResult  r  e    gt  new   msg   e Message  path   r Path             Handled               public void Configure IApplicationBuilder app  IHostingEnvironment env        app UseExceptionHandlingPolicies        app UseMvc

User · Answer

use middleware or IExceptionHandlerPathFeature is fine  there is another way in eshop  create a exceptionfilter and register it   public class HttpGlobalExceptionFilter   IExceptionFilter     public void OnException ExceptionContext context              services AddMvc options   gt      options Filters Add typeof HttpGlobalExceptionFilter

User · Answer

A simple way to handle an exception on any particular method is  using Microsoft AspNetCore Http       public ActionResult MyAPIMethod         try              var myObject       something          return Json myObject             catch  Exception ex                Log Error   quot Error   ex Message  quot            return StatusCode StatusCodes Status500InternalServerError

User · Answer

The well-accepted answer helped me a lot but I wanted to pass HttpStatusCode in my middleware to manage error status code at runtime  According to this link I got some idea to do the same  So I merged the Andrei Answer with this  So my final code is below  1  Base class public class ErrorDetails       public int StatusCode   get  set        public string Message   get  set         public override string ToString                 return JsonConvert SerializeObject this            2  Custom Exception Class Type public class HttpStatusCodeException   Exception       public HttpStatusCode StatusCode   get  set        public string ContentType   get  set        quot text plain quot        public HttpStatusCodeException HttpStatusCode statusCode                this StatusCode   statusCode             public HttpStatusCodeException HttpStatusCode statusCode  string message             base message                this StatusCode   statusCode             public HttpStatusCodeException HttpStatusCode statusCode  Exception inner             this statusCode  inner ToString             public HttpStatusCodeException HttpStatusCode statusCode  JObject errorObject             this statusCode  errorObject ToString                  this ContentType     quot application json quot              3  Custom Exception Middleware public class CustomExceptionMiddleware       private readonly RequestDelegate next       public CustomExceptionMiddleware RequestDelegate next                this next   next             public async Task Invoke HttpContext context    other dependencies                   try                       await next context                     catch  HttpStatusCodeException ex                        await HandleExceptionAsync context  ex                     catch  Exception exceptionObj                        await HandleExceptionAsync context  exceptionObj                        private Task HandleExceptionAsync HttpContext context  HttpStatusCodeException exception                string result   null          context Response ContentType    quot application json quot           if  exception is HttpStatusCodeException                        result   new ErrorDetails                                  Message   exception Message                  StatusCode    int exception StatusCode                ToString                context Response StatusCode    int exception StatusCode                    else                       result   new ErrorDetails                                   Message    quot Runtime Error quot                   StatusCode    int HttpStatusCode BadRequest               ToString                context Response StatusCode    int HttpStatusCode BadRequest                    return context Response WriteAsync result              private Task HandleExceptionAsync HttpContext context  Exception exception                string result   new ErrorDetails                           Message   exception Message              StatusCode    int HttpStatusCode InternalServerError            ToString            context Response StatusCode    int HttpStatusCode BadRequest          return context Response WriteAsync result            4  Extension Method public static void ConfigureCustomExceptionMiddleware this IApplicationBuilder app        app UseMiddleware lt CustomExceptionMiddleware gt         5  Configure Method in startup cs app ConfigureCustomExceptionMiddleware    app UseMvc     Now my login method in Account controller   try       IRepository lt UserMaster gt  obj            new Repository lt UserMaster gt   objHeaderCapture  Constants Tables UserMaster       var result   obj Get            AsQueryable            Where sb   gt  sb EmailId ToLower      objData UserName ToLower                 amp  amp  sb Password    objData Password ToEncrypt                 amp  amp  sb Status     int StatusType Active           FirstOrDefault        if  result    null   User Found         return result      else    Not Found         throw new HttpStatusCodeException HttpStatusCode NotFound               quot Please check username or password quot      catch  Exception ex        throw ex     Above you can see if i have not found the user then raising the HttpStatusCodeException in which i have passed HttpStatusCode NotFound status and a custom message  In middleware  catch  HttpStatusCodeException ex   blocked will be called which will pass control to  private Task HandleExceptionAsync HttpContext context  HttpStatusCodeException exception  method  But what if i got runtime error before  For that i have used try catch block which throw exception and will be catched in catch  Exception exceptionObj  block and will pass control to  Task HandleExceptionAsync HttpContext context  Exception exception   method  I have used a single ErrorDetails class for uniformity

User · Answer

If you want set custom exception handling behavior for a specific controller  you can do so by overriding the controllers OnActionExecuted method  Remember to set the ExceptionHandled property to true to disable default exception handling behavior  Here is a sample from an api I m writing  where I want to catch specific types of exceptions and return a json formatted result      private static readonly Type   API CATCH EXCEPTIONS   new Type                 typeof InvalidOperationException           typeof ValidationException                         public override void OnActionExecuted ActionExecutedContext context                base OnActionExecuted context            if  context Exception    null                        var exType   context Exception GetType                if  API CATCH EXCEPTIONS Any type   gt  exType    type    exType IsSubclassOf type                                  context Result   Problem detail  context Exception Message                   context ExceptionHandled   true

User · Answer

Use built-in Exception Handling Middleware Step 1  In your startup  register your exception handling route     It should be one of your very first registrations app UseExceptionHandler  quot  error quot       Add this app UseEndpoints endpoints   gt  endpoints MapControllers      Step 2  Create controller that will handle all exceptions and produce error response   ApiExplorerSettings IgnoreApi   true   public class ErrorsController   ControllerBase        Route  quot error quot        public MyErrorResponse Error                 var context   HttpContext Features Get lt IExceptionHandlerFeature gt             var exception   context Error     Your exception         var code   500     Internal Server Error by default          if       exception is MyNotFoundException  code   404     Not Found         else if  exception is MyUnauthException    code   401     Unauthorized         else if  exception is MyException          code   400     Bad Request          Response StatusCode   code     You can use HttpStatusCode enum instead          return new MyErrorResponse exception      Your error model          A few important notes and observations    ApiExplorerSettings IgnoreApi   true   is needed  Otherwise  it may break your Swashbuckle swagger Again  app UseExceptionHandler  quot  error quot    has to be one of the very top registrations in your Startup Configure      method  It s probably safe to place it at the top of the method  The path in app UseExceptionHandler  quot  error quot   and in controller  Route  quot error quot    should be the same  to allow the controller handle exceptions redirected from exception handler middleware   Microsoft documentation for this subject is not that great but has some interesting ideas  I ll just leave the link here  Response models and custom exceptions Implement your own response model and exceptions  This example is just a good starting point  Every service would need to handle exceptions in its own way  But with this code  you have full flexibility and control over handling exceptions and returning a proper result to the caller  An example of error response model  just to give you some ideas   public class MyErrorResponse       public string Type   get  set        public string Message   get  set        public string StackTrace   get  set         public MyErrorResponse Exception ex                Type   ex GetType   Name          Message   ex Message          StackTrace   ex ToString             For simpler services  you might want to implement http status code exception that would look like this  public class HttpStatusException   Exception       public HttpStatusCode Status   get  private set         public HttpStatusException HttpStatusCode status  string msg    base msg                Status   status           This can be thrown like that  throw new HttpStatusCodeException HttpStatusCode NotFound   quot User not found quot     Then your handling code could be simplified to  if  exception is HttpStatusException httpException        code    int  httpException Status      Why so un-obvious HttpContext Features Get lt IExceptionHandlerFeature gt     ASP NET Core developers embraced the concept of middlewares where different aspects of functionality such as Auth  Mvc  Swagger etc  are separated and executed sequentially by processing the request and returning the response or passing the execution to the next middleware  With this architecture  MVC itself  for instance  would not be able to handle errors happening in Auth  So  they came up with exception handling middleware that catches all the exceptions happening in middlewares registered down in the pipeline  pushes exception data into HttpContext Features  and re-runs the pipeline for specified route   error   allowing any middleware to handle this exception  and the best way to handle it is by our Controllers to maintain proper content negotiation

User · Answer

By adding your own  quot Exception Handling Middleware quot   makes it hard to reuse some good built-in logic of Exception Handler like send an  quot RFC 7807-compliant payload to the client quot  when an error happens  What I made was to extend built-in Exception handler outside of the Startup cs class to handle custom exceptions or override the behavior of existing ones  For example  an ArgumentException and convert into BadRequest without changing the default behavior of other exceptions  on the Startup cs add  app UseExceptionHandler  quot  error quot     and extend ErrorController cs with something like this  using System  using Microsoft AspNetCore Diagnostics  using Microsoft AspNetCore Hosting  using Microsoft AspNetCore Mvc  using Microsoft Extensions Hosting   namespace Api Controllers        ApiController       ApiExplorerSettings IgnoreApi   true        AllowAnonymous      public class ErrorController   ControllerBase                Route  quot  error quot            public IActionResult Error               FromServices  IWebHostEnvironment webHostEnvironment                        var context   HttpContext Features Get lt IExceptionHandlerFeature gt                 var exceptionType   context Error GetType                             if  exceptionType    typeof ArgumentException                     exceptionType    typeof ArgumentNullException                     exceptionType    typeof ArgumentOutOfRangeException                                 if  webHostEnvironment IsDevelopment                                          return ValidationProblem                          context Error StackTrace                          title  context Error Message                                      return ValidationProblem context Error Message                              if  exceptionType    typeof NotFoundException                                 return NotFound context Error Message                              if  webHostEnvironment IsDevelopment                                  return Problem                      context Error StackTrace                      title  context Error Message                                                               return Problem                       Note that   NotFoundException is a custom exception and all you need to do is throw new NotFoundException null   or throw new ArgumentException  quot Invalid argument  quot    You should not serve sensitive error information to clients  Serving errors is a security risk

User · Answer

There is a built-in middleware that makes it easier than writing a custom one  Asp Net Core 5 version  app UseExceptionHandler a   gt  a Run async context   gt        var exceptionHandlerPathFeature   context Features Get lt IExceptionHandlerPathFeature gt         var exception   exceptionHandlerPathFeature Error           await context Response WriteAsJsonAsync new   error   exception Message           Older versions  they did not have WriteAsJsonAsync extension   app UseExceptionHandler a   gt  a Run async context   gt        var exceptionHandlerPathFeature   context Features Get lt IExceptionHandlerPathFeature gt         var exception   exceptionHandlerPathFeature Error           var result   JsonConvert SerializeObject new   error   exception Message         context Response ContentType    quot application json quot       await context Response WriteAsync result         It should do pretty much the same  just a bit less code to write  Important  Remember to add it before UseMvc  or UseRouting in  Net Core 3  as order is important

User · Answer

Your best bet is to use middleware to achieve logging you re looking for   You want to put your exception logging in one middleware and then handle your error pages displayed to the user in a different middleware   That allows separation of logic and follows the design Microsoft has laid out with the 2 middleware components   Here s a good link to Microsoft s documentation  Error Handling in ASP Net Core  For your specific example  you may want to use one of the extensions in the StatusCodePage middleware or roll your own like this   You can find an example here for logging exceptions  ExceptionHandlerMiddleware cs  public void Configure IApplicationBuilder app           app UseErrorPage ErrorPageOptions ShowAll          app UseStatusCodePages           app UseStatusCodePages context   gt  context HttpContext Response SendAsync  Handler  status code      context HttpContext Response StatusCode   text plain            app UseStatusCodePages  text plain    Response  status code   0            app UseStatusCodePagesWithRedirects    errors  0            app UseStatusCodePagesWithRedirects   base errors  0            app UseStatusCodePages builder   gt  builder UseWelcomePage         app UseStatusCodePagesWithReExecute   Errors  0         I use this version         Exception handling logging below     app UseExceptionHandler        If you don t like that specific implementation  then you can also use ELM Middleware  and here are some examples  Elm Exception Middleware  public void Configure IApplicationBuilder app        app UseStatusCodePagesWithReExecute   Errors  0            Exception handling logging below     app UseElmCapture        app UseElmPage        If that doesn t work for your needs  you can always roll your own Middleware component by looking at their implementations of the ExceptionHandlerMiddleware and the ElmMiddleware to grasp the concepts for building your own   It s important to add the exception handling middleware below the StatusCodePages middleware but above all your other middleware components   That way your Exception middleware will capture the exception  log it  then allow the request to proceed to the StatusCodePage middleware which will display the friendly error page to the user

User · Answer

Firstly  thanks to Andrei as I ve based my solution on his example    I m including mine as it s a more complete sample and might save readers some time    The limitation of Andrei s approach is that doesn t handle logging  capturing potentially useful request variables and content negotiation  it will always return JSON no matter what the client has requested - XML   plain text etc     My approach is to use an ObjectResult which allows us to use the functionality baked into MVC    This code also prevents caching of the response   The error response has been decorated in such a way that it can be serialized by the XML serializer     public class ExceptionHandlerMiddleware       private readonly RequestDelegate next      private readonly IActionResultExecutor lt ObjectResult gt  executor      private readonly ILogger logger      private static readonly ActionDescriptor EmptyActionDescriptor   new ActionDescriptor         public ExceptionHandlerMiddleware RequestDelegate next  IActionResultExecutor lt ObjectResult gt  executor  ILoggerFactory loggerFactory                this next   next          this executor   executor          logger   loggerFactory CreateLogger lt ExceptionHandlerMiddleware gt                public async Task Invoke HttpContext context                try                       await next context                     catch  Exception ex                        logger LogError ex    An unhandled exception has occurred while executing the request  Url   context Request GetDisplayUrl     Request Data      GetRequestData context                 if  context Response HasStarted                                throw                             var routeData   context GetRouteData      new RouteData                 ClearCacheHeaders context Response                var actionContext   new ActionContext context  routeData  EmptyActionDescriptor                var result   new ObjectResult new ErrorResponse  Error processing request  Server error                                   StatusCode    int  HttpStatusCode InternalServerError                              await executor ExecuteAsync actionContext  result                        private static string GetRequestData HttpContext context                var sb   new StringBuilder             if  context Request HasFormContentType  amp  amp  context Request Form Any                          sb Append  Form variables                 foreach  var x in context Request Form                                sb AppendFormat  Key  0   Value  1  lt br  gt    x Key  x Value                                    sb AppendLine  Method      context Request Method            return sb ToString               private static void ClearCacheHeaders HttpResponse response                response Headers HeaderNames CacheControl     no-cache           response Headers HeaderNames Pragma     no-cache           response Headers HeaderNames Expires     -1           response Headers Remove HeaderNames ETag               DataContract Name   ErrorResponse        public class ErrorResponse                DataMember Name    Message            public string Message   get  set             public ErrorResponse string message                        Message   message

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