Is there a way to check if exception is handled on a higher application level to skip logging and re-throw? Like this, for example:
try
{
// Execute some code
}
catch (Exception e)
{
if(!ExceptionIsHandled())
LogError(e);
throw e;
}
Nothing that I'm aware of. If you're committed to this design (see note at end), you could write a wrapper for an Exception that's some sort of HandledException and just make its InnerException be the one that was thrown. Then you could make your code look like:
try
{
// Execute some code
}
catch (HandledException e)
{
LogError(e.InnerException);
// Do something else
}
catch (Exception e)
{
throw ;
}
Here comes the stereotypical Stackoverflow "you're doin it wrong" part of the answer...
However, if you've truly "handled" the exception, it doesn't make a lot of sense to be re-throwing it. Maybe your method should just return a failure result, possibly including the Exception as a detail item for what went wrong.
This is old, but I do have some input here. There is a design pattern I've used before that does this very well, but does add a little bit of overhead to everything.
Basically, all methods would return a response object (e.g., Response<T>). Any exceptions that occur should be wrapped in the response object and returned instead of thrown.
public class Response<T>
{
public T Payload { get; set; }
public bool IsSuccessful { get; set; } = false;
public string Message { get; set; }
public Exception Error { get; set; }
}
public class MyService
{
public Response<IEnumerable<Customer>> GetCustomers()
{
var response = new Response<IEnumerable<Customer>>();
try
{
var customers = new List<Customer>()
{
new Customer() { CompanyName = "ABC Co." },
new Customer() { CompanyName = "ACME" }
};
response.Payload = customers;
response.IsSuccessful = true;
}
catch (Exception e)
{
response.IsSuccessful = false;
response.Error = e;
// A friendly message, safe to show to users.
response.Message = "An error occurred while attempting to retrieve customers.";
}
return response;
}
}
You can bubble up the exception without rethrowing it, and handle appropriately. You can then add exception catches for more custom user-friendly messages.
I also use a custom base Exception type for any errors that are safe to show the client. This way I can add a generic catch at the controller level to propagate those prepared error messages.
Well no, hasn't got there yet has it. Exceptions bubble up through handlers.
Usual way to go about this.
Is define your own exceptions, then only catch the ones you are going to handle where you are.
If you could be certain that code was wrapped within a specially-designed try-catch block which was written in a language that supports exception filters, it would be possible to determine before or during stack unwinding whether the exception was likely to be caught by that outer block or by an inner one. The usefulness of this is rather limited, however, especially given the extremely common anti-pattern of code catching and rethrowing exceptions that it knows it's not going to resolve, simply for the purpose of finding out that they occurred.
If your goal is simply to avoid redundant logging, I'd suggest that you should use a logging facility which can deal efficiently with redundancy. While some people might argue that it's better to have exceptions logged just once at the outer layers, there are advantages to having more logging opportunities. If an exception occurs within the inner layer and a middle layer swallows it, logging code within the outer layer will never find out about it. By contrast, if the inner layer starts out by capturing the exception and arranging for it to get logged, then even if the middle layer swallows the exception the fact that it occurred could still get recorded.
Related
I have seen similar questions, but not exactly this:
I would like to know the right way of determining whether a method is executed correctly or not, returning a boolean, and if the method is not executed know the reason, even if an exception is thrown.
I do it in this way, but I think that return inside the catch is a bad practice, so which is the right way?:
if(!myObject.DoSomething('A', out result))
{
MessageBox.Show(myObject.ErrorMessage);
[...]
}else{
MessageBox.Show(result);
[...]
}
class myObject()
{
public string ErrorMessage;
bool DoSomething(char inputValue, out string result)
{
try
{
if(inputValue == 'A')
{
ErrorMessage = "Bad input value: " + inputValue;
return false;
}
[...]
return true;
}catch(Exception ex){
ErrorMessage = ex.Message;
return false;
}
}
I don't like trhow the exception inside the catch because I lose the control of the application (and I can't get the description), and the exception always finish in the form. And if I show the exception in the form, I don't need try catch in the rest of the classes.
I mean that try {} catch(Exception ex) { throw ex;} is the same as not putting try catch.
thanks a lot
My suggestion would be to create your own Exception type (possibly global), and pass it in as a reference.
Thereafter you can still get back your boolean indicating success or failure (and having only one return outside of the try..catch).
public class CustomException
{
private string _message;
private string _title;
public CustomException()
{
_title = "";
_message = "";
}
public CustomException(string title, string message)
{
_title = title;
_message = message;
}
}
Then call DoSomething passing in an instance of CustomException (ce in this case).
CustomException ce = new CustomException();
Be advised this is the best process to solve the problem of having to return a boolean indicating success or failure and know the message, for example; dumping it to a log file or logging to database (particularly for Service Calls - WCF)
However this is not a solution for bad logic in handling business process.
Return false inside a catch isn't by itself bad practice. It's useful when you handle a piece of code's exceptions and it must not fail.
For example, I'm working on a printer piloting DLL at the time, and this DLL must read a XML file containing multiple records to print. The method must not fail because one record fails to print, but it still can return exception if the XML file is not correctly formated.
public void Print(string xmlFile)
{
if (String.IsNullOrWhiteSpace(xmlFile))
throw new ArgumentNullException("No xml file has been passed to the Print method.");
// This line will most likely throw an exception if the XMl file is not well formated
XDocument dom = XDocument.Load(xmlFile);
foreach (XElement n in dom.XPathSelectElements("//RECORDS/RECORD"))
{
try
{
// send commands to the printer, if the printer fails to print, throw a PrinterRecordException
}
catch (PrinterRecordException e)
{
// log print failure, but keep on printing the rest
continue;
}
catch (Exception e)
{
// dunno what happened, but still have to print the rest
continue;
}
}
}
In this example, my function could return false instead of throwing exceptions to the main program, if this program doesn't care. In my case it does :p In my opinion, that's how you should think your method.
Exception handling methods and best practices are a some-what subjective matter. I cannot attest to the method I'm about to present because I have only just started to use it in my own project.
What I suggest is having a static ExceptionHandler class with which you can register any exception to be handled by Generic Parameter and its corresponding handler. This will decouple your business logic from your UI in case you wanted to display some kind of message box when a particular exception occurs.
Here's an example:
/// the real implementation uses lambda's and/or implementations of IExceptionHandler<TException>
ExceptionHandler.Register<InvalidPasswordException>(() => /*some handler logic*/);
// ... else where in the code ...
catch (InvalidPasswordException ex)
{
// do resource clean-up and raise exception for listeners such as the UI or logging infrastructure.
ExceptionHandler.Raise(ex);
}
So far this looks promising, especially when compared with my previous approaches. But only time will tell.
Update
The ExceptionHandler class itself need not be static, for example you might want to have different instances of ExceptionHandlers at different layers of your application if you are using a layered architecture.
I am building a ASP.NET Web Api service and I would like to create centralized exception handling code.
I want to handle different types of exceptions in different ways. I will log all exceptions using log4net. For some types of exceptions I will want to notify an administrator via email. For some types of exceptions I want to rethrow a friendlier exception that will be returned to the caller. For some types of exceptions I want to just continue processing from the controller.
But how do I do that? I am using an Exception Filter Attribute. I have this code working. The attribute is registered properly and the code is firing. I just want to know how I can continue if certain types of exceptions are thrown. Hope that makes sense.
public class MyExceptionHandlingAttribute : ExceptionFilterAttribute
{
public override void OnException(HttpActionExecutedContext actionExecutedContext)
{
//Log all errors
_log.Error(myException);
if(myException is [one of the types I need to notify about])
{
...send out notification email
}
if(myException is [one of the types that we continue processing])
{
...don't do anything, return back to the caller and continue
...Not sure how to do this. How do I basically not do anything here?
}
if(myException is [one of the types where we rethrow])
{
throw new HttpResponseException(new HttpResponseMessage(StatusCode.InternalServerError)
{
Content = new StringContent("Friendly message goes here."),
ReasonPhrase = "Critical Exception"
});
}
}
}
For some types of exceptions I want to just continue processing from the controller. But how do I do that?
By writing try..catch where you want this behaviour to occur. See Resuming execution of code after exception is thrown and caught.
To clarify, I assume you have something like this:
void ProcessEntries(entries)
{
foreach (var entry in entries)
{
ProcessEntry(entry);
}
}
void ProcessEntry(entry)
{
if (foo)
{
throw new EntryProcessingException();
}
}
And when EntryProcessingException is thrown, you actually don't care and want to continue execution.
If this assumption is correct: you can't do that with a global exception filter, as once an exception is caught, there's no returning execution to where it was thrown. There is no On Error Resume Next in C#, especially not when the exceptions are handled using filters as #Marjan explained.
So, remove EntryProcessingException from your filter, and catch that specific exception by changing the loop body:
void ProcessEntries(entries)
{
foreach (var entry in entries)
{
try
{
ProcessEntry(entry);
}
catch (EntryProcessingException ex)
{
// Log the exception
}
}
}
And your loop will happily spin to its end, but throw on all other exceptions where it will be handled by your filter.
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)
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.
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.
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.
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
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:
An ID number that will help me find the location in the code where the problem occurred.
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.
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.
Anything else that might be useful given the circumstance.
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.
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.
I have an MVC EF5 setup, with classes:
Program - this is the controller
UserInterface - this is the view, responsible for displaying and prompting for data.
DataAccess - Model, this Creates, Reads, Updates, and Deletes data in my EF model classes
When the DataAccess class tries to do a CRUD operation on my database, if it catches an error, it needs to be handled, my UserInterface class needs to print messages to the user, reporting any errors if neccessary. So, when an error happens, it needs to go through the program class first, then to the UserInterface class, because data layer shouldn't directly communicate to the presentation layer.
It was suggested to me that I don't pass or return the exception to a calling function, but that I should "throw a new simpler exception to the layers above". All this talk about exceptions is confusing to me because My experience with exceptions is limited to this format:
try
{
// stuff
}
catch (exception ex)
{
console.writeline(ex.ToString());
}
I've done some of my own research to try and find the answer to this problem, and I've learned a few things but don't know how to put it all together:
I learned:
throw; rethrows an exception and preserves the stack trace
throw ex throws an existing exception, such as one caught in a catch block. and resets the stack trace.
There is a property called Exception.StackTrace. I understand that each time an exception is thrown, the frames in the call stack are recorded to the Exception.StackTrace property.
However, I don't know where to place my try/catch blocks to utilize rethrowing
Is it something like the following code? Or am I missing the point on how this works?
EDITED: (added a little more to make sense of this guesswork to others)
void MethodA()
{
try
{
MethodB();
}
catch (MyExceptionType ex)
{
// Do stuff appropriate for MyExceptionType
throw;
}
}
void MethodB()
{
try
{
MethodC();
}
catch (AnotherExceptionType ex)
{
// Do stuff appropriate for AnotherExceptionType
throw;
}
}
void MethodC()
{
try
{
// Do Stuff
}
catch (YetAnotherExceptionType ex)
{
// Do stuff appropriate for YetAnotherExceptionType
throw;
}
}
There is more than how you use different type of exception handling. Functionally you should define what layers has to do what with a exception.
Like data layer => dont throw anything other than DataException or SQLException. Log them and throw back a generic database exception back to UI.
Business layer => log and rethrow simple bussiness exception
UI layer => catch only business exception and alert it in a message inside business exception
Once all this is defined, you can use what you have learned and summarized in question to build this.
What (I think) was suggested you do by throw a new simpler exception is that you translate the exceptions from the lower layers into new, higher level exceptions for consuming in the outer layers. The lower level exceptions are not suitable for consumption at the upper levels of the program.
For example, in LINQ to Entities, the method Single() will throw an InvalidOperationException when the sequence has no elements. However, this exception type is very common, so catching it in the user interface levels is hard to do: how would you differentiate between different possibilities of this exception being thrown (for example, modifying a read-only collection)? The solution is to translate the exception into another (new, user-defined) type that the application can easily handle.
Here is a simple example of the idea:
public class MyUserService {
public User GetById(int id) {
try {
using(var ctx = new ModelContainer()) {
return ctx.Where(u => u.Id == id).Single();
}
}
catch(InvalidOperationException) {
// OOPs, there is no user with the given id!
throw new UserNotFoundException(id);
}
}
}
Then the Program layer can catch the UserNotFoundException and know instantly what happened, and thus find the best way to explain the error to the user.
The details will depend on the exact structure of your program, but something like this would work in an ASP.NET MVC app:
public class MyUserController : Controller {
private MyUserService Service = new MyUserService();
public ActionResult Details(int id) {
User user;
try {
user = Service.GetById(id);
}
catch(UserNotFoundException) {
// Oops, there is no such user. Return a 404 error
// Note that we do not care about the InvalidOperationException
// that was thrown inside GetById
return HttpNotFound("The user does not exist!");
}
// If we reach here we have a valid user
return View(user);
}
}
Situation:
My application need to process the first step in the business rules (the initial try-catch statement). If an certain error occurs when the process calls the helper method during the step, I need to switch to a second process in the catch statement. The back up process uses the same helper method. If an same error occurs during the second process, I need to stop the entire process and throw the exception.
Implementation:
I was going to insert another try-catch statement into the catch statement of the first try-catch statement.
//run initial process
try
{
//initial information used in helper method
string s1 = "value 1";
//call helper method
HelperMethod(s1);
}
catch(Exception e1)
{
//backup information if first process generates an exception in the helper method
string s2 = "value 2";
//try catch statement for second process.
try
{
HelperMethod(s2);
}
catch(Exception e2)
{
throw e2;
}
}
What would be the correct design pattern to avoid code smells in this implementation?
I caused some confusion and left out that when the first process fails and switches to the second process, it will send different information to the helper method. I have updated the scenario to reflect the entire process.
If the HelperMethod needs a second try, there is nothing directly wrong with this, but your code in the catch tries to do way too much, and it destroys the stacktrace from e2.
You only need:
try
{
//call helper method
HelperMethod();
}
catch(Exception e1)
{
// maybe log e1, it is getting lost here
HelperMethod();
}
I wouldn't say it is bad, although I'd almost certainly refactor the second block of code into a second method, so keep it comprehensible. And probably catch something more specific than Exception. A second try is sometimes necessary, especially for things like Dispose() implementations that might themselves throw (WCF, I'm looking at you).
The general idea putting a try-catch inside the catch of a parent try-catch doesn't seem like a code-smell to me. I can think of other legitimate reasons for doing this - for instance, when cleaning up an operation that failed where you do not want to ever throw another error (such as if the clean-up operation also fails). Your implementation, however, raises two questions for me: 1) Wim's comment, and 2) do you really want to entirely disregard why the operation originally failed (the e1 Exception)? Whether the second process succeeds or fails, your code does nothing with the original exception.
Generally speaking, this isn't a problem, and it isn't a code smell that I know of.
With that said, you may want to look at handling the error within your first helper method instead of just throwing it (and, thus, handling the call to the second helper method in there). That's only if it makes sense, but it is a possible change.
Yes, a more general pattern is have the basic method include an overload that accepts an int attempt parameter, and then conditionally call itself recursively.
private void MyMethod (parameterList)
{ MyMethod(ParameterList, 0)l }
private void MyMethod(ParameterList, int attempt)
{
try { HelperMethod(); }
catch(SomeSpecificException)
{
if (attempt < MAXATTEMPTS)
MyMethod(ParameterList, ++attempt);
else throw;
}
}
It shouldn't be that bad. Just document clearly why you're doing it, and most DEFINITELY try catching a more specific Exception type.
If you need some retry mechanism, which it looks like, you may want to explore different techniques, looping with delays etc.
It would be a little clearer if you called a different function in the catch so that a reader doesn't think you're just retrying the same function, as is, over again. If there's state happening that's not being shown in your example, you should document it carefully, at a minimum.
You also shouldn't throw e2; like that: you should simply throw; if you're going to work with the exception you caught at all. If not, you shouldn't try/catch.
Where you do not reference e1, you should simply catch (Exception) or better still catch (YourSpecificException)
If you're doing this to try and recover from some sort of transient error, then you need to be careful about how you implement this.
For example, in an environment where you're using SQL Server Mirroring, it's possible that the server you're connected to may stop being the master mid-connection.
In that scenario, it may be valid for your application to try and reconnect, and re-execute any statements on the new master - rather than sending an error back to the caller immediately.
You need to be careful to ensure that the methods you're calling don't have their own automatic retry mechanism, and that your callers are aware there is an automatic retry built into your method. Failing to ensure this can result in scenarios where you cause a flood of retry attempts, overloading shared resources (such as Database servers).
You should also ensure you're catching exceptions specific to the transient error you're trying to retry. So, in the example I gave, SqlException, and then examining to see if the error was that the SQL connection failed because the host was no longer the master.
If you need to retry more than once, consider placing an 'automatic backoff' retry delay - the first failure is retried immediately, the second after a delay of (say) 1 second, then doubled up to a maximum of (say) 90 seconds. This should help prevent overloading resources.
I would also suggest restructuring your method so that you don't have an inner-try/catch.
For example:
bool helper_success = false;
bool automatic_retry = false;
//run initial process
try
{
//call helper method
HelperMethod();
helper_success = true;
}
catch(Exception e)
{
// check if e is a transient exception. If so, set automatic_retry = true
}
if (automatic_retry)
{ //try catch statement for second process.
try
{
HelperMethod();
}
catch(Exception e)
{
throw;
}
}
Here's another pattern:
// set up state for first attempt
if(!HelperMethod(false)) {
// set up state for second attempt
HelperMethod(true);
// no need to try catch since you're just throwing anyway
}
Here, HelperMethod is
bool HelperMethod(bool throwOnFailure)
and the return value indicates whether or not success occurred (i.e., false indicates failure and true indicates success). You could also do:
// could wrap in try/catch
HelperMethod(2, stateChanger);
where HelperMethod is
void HelperMethod(int numberOfTries, StateChanger[] stateChanger)
where numberOfTries indicates the number of times to try before throwing an exception and StateChanger[] is an array of delegates that will change the state for you between calls (i.e., stateChanger[0] is called before the first attempt, stateChanger[1] is called before the second attempt, etc.)
This last option indicates that you might have a smelly setup though. It looks like the class that is encapsulating this process is responsible for both keeping track of state (which employee to look up) as well as looking up the employee (HelperMethod). By SRP, these should be separate.
Of course, you need to a catch a more specific exception than you currently are (don't catch the base class Exception!) and you should just throw instead of throw e if you need to rethrow the exception after logging, cleanup, etc.
You could emulate C#'s TryParse method signatures:
class Program
{
static void Main(string[] args)
{
Exception ex;
Console.WriteLine("trying 'ex'");
if (TryHelper("ex", out ex))
{
Console.WriteLine("'ex' worked");
}
else
{
Console.WriteLine("'ex' failed: " + ex.Message);
Console.WriteLine("trying 'test'");
if (TryHelper("test", out ex))
{
Console.WriteLine("'test' worked");
}
else
{
Console.WriteLine("'test' failed: " + ex.Message);
throw ex;
}
}
}
private static bool TryHelper(string s, out Exception result)
{
try
{
HelperMethod(s);
result = null;
return true;
}
catch (Exception ex)
{
// log here to preserve stack trace
result = ex;
return false;
}
}
private static void HelperMethod(string s)
{
if (s.Equals("ex"))
{
throw new Exception("s can be anything except 'ex'");
}
}
}
Another way is to flatten the try/catch blocks, useful if you're using some exception-happy API:
public void Foo()
{
try
{
HelperMethod("value 1");
return; // finished
}
catch (Exception e)
{
// possibly log exception
}
try
{
HelperMethod("value 2");
return; // finished
}
catch (Exception e)
{
// possibly log exception
}
// ... more here if needed
}
An option for retry (that most people will probably flame) would be to use a goto. C# doesn't have filtered exceptions but this could be used in a similar manner.
const int MAX_RETRY = 3;
public static void DoWork()
{
//Do Something
}
public static void DoWorkWithRetry()
{
var #try = 0;
retry:
try
{
DoWork();
}
catch (Exception)
{
#try++;
if (#try < MAX_RETRY)
goto retry;
throw;
}
}
In this case you know this "exception" probably will happen so I would prefer a simple approach an leave exceptions for the unknown events.
//run initial process
try
{
//initial information used in helper method
string s1 = "value 1";
//call helper method
if(!HelperMethod(s1))
{
//backup information if first process generates an exception in the helper method
string s2 = "value 2";
if(!HelperMethod(s2))
{
return ErrorOfSomeKind;
}
}
return Ok;
}
catch(ApplicationException ex)
{
throw;
}
I know that I've done the above nested try catch recently to handle decoding data where two third party libraries throw exceptions on failure to decode (Try json decode, then try base64 decode), but my preference is to have functions return a value which can be checked.
I generally only use the throwing of exceptions to exit early and notify something up the chain about the error if it's fatal to the process.
If a function is unable to provide a meaningful response, that is not typically a fatal problem (Unlike bad input data).
It seems like the main risk in nested try catch is that you also end up catching all the other (maybe important) exceptions that might occur.