When to Throw an Exception? When to Handle an Exception (Service Layer, Controller) where?

Question

I rewritten my question as I think it was too wordy and maybe what I am trying to achieve was lost.

I written this code in notepad so it may have mistakes and some stuff maybe not well thoughout but it is to illustrate what I see my options are.

// I wrap all code send back from service layer to controller in this class.
  public class ResponseResult
    {

        public ResponseResult()
        {
            Errors = new Dictionary<string, string>();
            Status = new ResponseBase();
        }

        public void AddError(string key, string errorMessage)
        {
            if (!Errors.ContainsKey(key))
            {
                Errors.Add(key, errorMessage);
            }
        }

        public bool IsValid()
        {
            if (Errors.Count > 0)
            {
                return false;
            }

            return true;
        }


        public Dictionary<string, string> Errors { get; private set; }


        public ResponseBase Status { get; set; }
    }

    public class ResponseResult<T> : ResponseResult
    {

        public T Response { get; set; }
    }

    public class ResponseBase
    {
        public HttpStatusCode Code { get; set; }
        public string Message { get; set; }
    }

Option 1 (what I am using now)

//controller
    public HttpResponseMessage GetVenue(int venueId)
            {
                if (venueId == 0)
                {
                    ModelState.AddModelError("badVenueId", "venue id must be greater than 0");

                if (ModelState.IsValid)
                {
                    var venue = venueService.FindVenue(venueId);
                    return Request.CreateResponse<ResponseResult<Venue>>(venue.Status.Code, venue);
                }

                // a wrapper that I made to extract the model state and try to make all my request have same layout.
                var responseResult = new ResponseResultWrapper();
                responseResult.Status.Code = HttpStatusCode.BadRequest;
                responseResult.Status.Message = GenericErrors.InvalidRequest;
                responseResult.ModelStateToResponseResult(ModelState);

                return Request.CreateResponse<ResponseResult>(responseResult.Status.Code, responseResult);
            }

// service layer        
             public ResponseResult<Venue> FindVenue(int venueId)
            {
                ResponseResult<Venue> responseResult = new ResponseResult<Venue>();

                try
                {
                    // I know this check was done in the controller but pretend this is some more advanced business logic validation.
                    if(venueId == 0)
                    {
                       // this is like Model State Error in MVC and mostly likely would with some sort of field.
                       responseResult.Errors.Add("badVenueId", "venue id must be greater than 0");
                       responseResult.Status.Code = HttpStatusCode.BadRequest;
                    }

                    var venue = context.Venues.Where(x => x.Id == venueId).FirstOrDefault();

                    if(venue == null)
                    {
                        var foundVenue = thirdPartyService.GetVenue(venueId);

                        if(foundVenue == null)
                        {
                           responseResult.Status.Code = HttpStatusCode.NotFound;
                           responseResult.Status.Message = "Oops could not find Venue";

                           return responseResult;
                        }
                        else
                        {
                           var city = cityService.FindCity(foundVenue.CityName);

                           if(city == null)
                           { 
                              city = cityService.CreateCity(foundVenue.CityName);

                              if(city.Response == null)
                              {
                                 responseResult.Status.Code = city.Status.Code;
                                 responseResult.Status.Message = city.Status.Message;

                                 return responseResult;
                              }

                              CreateVenue(VenueId, city.Response, foundVenue.Name);

                               responseResult.Status.Code = HttpStatusCode.Ok;
                               // I don't think I would return a success message here as the venue being displayed back to the user should be good enough.
                               responseResult.Status.Message = "";

                               reponseResult.Response = foundVenue;
                           }
                        }

                        return responseResult;
                    }

                }
                catch (SqlException ex)
                {
                    ErrorSignal.FromCurrentContext().Raise(ex);
                    responseResult.Status.Code = HttpStatusCode.InternalServerError;
                    responseResult.Status.Message = GenericErrors.InternalError;

                    // maybe roll back statement here depending on the method and what it is doing.
                }
               // should I catch this, I know it should be if you handle it but you don't want nasty messages going back to the user.
                catch (InvalidOperationException ex)
                {
                    ErrorSignal.FromCurrentContext().Raise(ex);
                    responseResult.Status.Code = HttpStatusCode.InternalServerError;
                    responseResult.Status.Message = GenericErrors.InternalError;
                }
               // should I catch this, I know it should be if you handle it but you don't want nasty messages going back to the user.
                catch (Exception ex)
                {
                    ErrorSignal.FromCurrentContext().Raise(ex);
                    responseResult.Status.Code = HttpStatusCode.InternalServerError;
                    responseResult.Status.Message = GenericErrors.InternalError;
                }

                return responseResult;
            }

// another service layer. 

        // it is ResponseResult<City> and not city because I could have a controller method that directly calls this method.
            // but I also have a case where my other method in another service needs this as well.
            public ResponseResult<City> CreateCity(string CityName)
            {
               ResponseResult<City> responseResult = new ResponseResult<City>();
               try
               {
                   City newCity = new City {  Name = "N" };
                   context.Cities.Add(newCity);
                   context.SaveChanges();

                    responseResult.Status.Code = HttpStatusCode.Ok;
                    responseResult.Status.Message = "City was succesfully added";
               }           
               // same catch statmens like above
               catch (SqlException ex)
                {
                    ErrorSignal.FromCurrentContext().Raise(ex);
                    responseResult.Status.Code = HttpStatusCode.InternalServerError;
                    responseResult.Status.Message = GenericErrors.InternalError;

                    // maybe roll back statement here depending on the method and what it is doing.
                }
                return responseResult;
            }

As you can see the methods are all wrapped in the status codes as they could be directly called by the controller being public. FindCity() and CreateVenue() could also have this wrapping.

Option 2

   public HttpResponseMessage GetVenue(int venueId)
        {
            try
            {
                if (venueId == 0)
                {
                    ModelState.AddModelError("badVenueId", "venue id must be greater than 0");

                if (ModelState.IsValid)
                {
                    var venue = venueService.FindVenue(venueId);
                    return Request.CreateResponse<ResponseResult<Venue>>(HttpSatusCode.Ok, venue);
                }

                // a wrapper that I made to extract the model state and try to make all my request have same layout.
                var responseResult = new ResponseResultWrapper();
                responseResult.Status.Code = HttpStatusCode.BadRequest;
                responseResult.Status.Message = GenericErrors.InvalidRequest;
                responseResult.ModelStateToResponseResult(ModelState);

                return Request.CreateResponse<ResponseResult>(responseResult.Status.Code, responseResult);
            }
            catchcatch (SqlException ex)
            {
               // can't remember how write this and too tried to look up.
               return Request.CreateResponse(HttpStatusCode.InternalServerError;, "something here");
            }
        }

 public Venue FindVenue(int venueId)
        {
            try
            {
                // how to pass back business logic error now without my wrapper?
                if(venueId == 0)
                {
                   // what here?
                }

                var venue = context.Venues.Where(x => x.Id == venueId).FirstOrDefault();

                if(venue == null)
                {
                    var foundVenue = thirdPartyService.GetVenue(venueId);

                    if(foundVenue == null)
                    {
                       // what here?
                    }
                    else
                    {
                       var city = cityService.FindCity(foundVenue.CityName);

                       if(city == null)
                       { 
                          city = cityService.CreateCity(foundVenue.CityName);

                          if(city  == null)
                          {
                             // what here?
                          }

                          CreateVenue(VenueId, city.Response, foundVenue.Name);


                       }
                    }

                    return venue;
                }

            }
            catch (SqlException ex)
            {
                // should there be a try catch here now? 
                // I am guessing I am going to need to have this here if I need to do a rollback and can't do it in the controller

                // throw exception here. Maybe this won't exist if no rollback is needed.
            }
            return null;
        }

        public City CreateCity(string CityName)
        {
           // if it crashes something I guess will catch it. Don't think I need to rollback here as only one statement being sent to database.
            City newCity = new City {  Name = "N" };
            context.Cities.Add(newCity);
            context.SaveChanges();

            return newCity;            
        }

As you see with option 2, I might still need to wrap it in try catches for rollbacks and I am not sure how to handle advanced business validation.

Also with catching everything in the controller and sending back vanilla objects(without my wrapper) I am unsure how to do fine grain HttpStatus codes(say like notFound,Create and such)

Answer 1

Sorry for the brief response, but here is my general rule - if an exception occurs which you expect might happen, deal with it - either by retrying or telling the user something went wrong and giving them options to fix it.

If an unexpected exception occurs, if it's something you can deal with (e.g a timeout which you can retry) try to deal with it, otherwise get out - just think what any MS app does - e.g. office - you get an apology that something went wrong and the app ends. It's better to end gracefully than to potentially corrupt data and leave things in a real mess.

Answer 2

This is an article with Java-specific concepts and examples, but the broad principles here are the way to go.

Distinguish between fault exceptions, which are catastrophic and unrecoverable, and contingency exceptions, which are very much recoverable. Let the faults "bubble" to the fault barrier, where you handle appropriately. For example, you might log the error, E-mail someone or send a message to a message queue, and present the user with a nice, informative error page.

Whatever you do, be sure to preserve all the exception information from the source.

Hope that helps.

Answer 3

Throw an exception wherever your code determines that something has gone wrong.

You always need to handle exceptions in methods which are called directly by the end-user. This is to cater for unexpected errors which your code doesn't have specific handling for. Your generic handling code would typically log the error and may or may not include letting the user know that an unexpected error has occurred.

But if there are errors which you can expect ahead of time, you'll often want to handle these lower down in the code, nearer to the point at which they occur, so that your application can "recover" from the error and continue.

Answer 4

I think exceptions are useful any time you need to return details of a failure from a method, whilst being able to use the ideal return type for the method you're calling.

You said in your question:

Now for me I try to return error messages back to the the controller and try not to really catch anything in the controller.

If the service method is supposed to ideally return a Venue object, how do you return this potential error message back to the controller? an out parameter? change the return type to something which has an error message property on it?

If you're doing either of those options, I think you're reinventing the wheel... i.e. creating a way to return exception information when one already exists.

Finally, Exceptions are strongly typed representations of what went wrong. If you return an error message, then that is fine to send back to the user, but if you need to programatically do different things based on the details of the error, then you don't want to be switching on magic string.

For example, wouldn't it be handy to differentiate between authorization errors and not found errors so you can return the most appropriate http status code to the user?

Don't forget that the Exception class has a Message property you can simply return to the user if you want to use it that way

Answer 5

To make sure I understand the question, your are creating a web service and want to know when to handle and when to throw exceptions.

In this situation I would strongly recommend that you catch all exceptions. "Unhandled" exceptions are very bad form. On web sites they result in displays that range from meaningless to dangerous by exposing internal information that you do no want the public to see.

If this is a good sized program I suggest that you create your own MyException class which derives from System.Exception. The purpose of this is provide a place for you to add additional information specific to your application. Here are some typical things I like to add to my MyException classes:

1. An ID number that will help me find the location in the code where the problem occurred.
2. A "LogMessage" method that logs the exception, sometimes to the Windows Event Log. Whether or not you log and to which log you write depends on what you want recorded, and the severity of the situation.
3. An indicator that shows the exception has been logged so the above method will not log twice even if it gets called more than once.
4. Anything else that might be useful given the circumstance.
5. I also like to put the text of the messages in an external resource file, like an XML document, and key them to the error number that you assign. This allows you to change the error text to improve clarity without having to redeploy the application.

Catch all exceptions and create a new instance of your MyException type and put the original exception into inner exception property. Below the first level of my application, I always throw one of my MyException instances rather than the original exception.

At the top level (application level), NEVER let an exception go unhandled and never throw your own exception. A better way is to return an error code and message in your data contract. That way the client application will only get what you want them to see. The only exceptions they'll need to worry about are the ones outside your scope, i.e. configuration errors or communication failures. In other words if they are able to invoke your service and the network stays connected you should give them a response they can interpret.

Hope this helps.

PS I didn't include a sample exception as I am sure a little searching will find many. Post if you want me to put up a simple sample.

Answer 6

Use try catch at all levels and bubble it up. Optionally, log the error in a file or database. I use text file - tab delimited. Capture at each level 1. Module Name (Use C# supplied methods to get this) 2. Method Name 3. Code Being Executed (User created - "Connecting to database") 4. Error Number 5. Error Description 6. Code Being Executed (User created - "Accessing database") 7. Error Number for the end user 8. Error Description for the end user Additionally, I also pass a unique identifier like - Session Id in case of Web, Logged in User Id, User Name (if available)

I always have the Exception catch block. In here I set the error number as -0 and the message from the exception object as the error description. If it is SQL Server related - I capture SQL Exception. This generates an error number - I use that.

I want to extend this some more though.