I have a C# console project where I have separated out the business logic from the UI. The business logic calls an API, retrieves JSON data, and then updates a database with the data. The UI handles the display of the results and also loops through a queue to process records with the business logic.
My question is how to properly handle exceptions thrown by the business logic. The UI project currently handles exceptions that bubble up but I want to give as detailed an error message as possible. For example, sometimes the API fails to authorize or may be down and I want to log that specific exception. The problem is the UI project does not know anything about HttpRequestException without me adding a reference to the System.Net.Http library, which creates a dependency I don't want.
What is the "best practice" for handling exceptions in a project other than where they are generated?
If you want to pass detailed messages to the UI project without explicitly referencing System.Net.Http, consider catching these HTTP exceptions in the business logic, wrapping them in an exception type you define in that library, and re-throwing the new exception. Then your UI library only needs to know about the business logic library, which it's already referencing, and the business logic can provide the most informative message possible for things it can't properly recover from.
I'd say the best practice is to not handle them. You don't know what those exceptions are, so how can you "handle" them?
Either trust the Message property of these exceptions and display them to the users, or just tell the users "sorry, something bad happened".
Keep in mind that your users will need to read the detailed messages and decide what to do about them. Quite likely they'll either ignore the messages or else call someone to ask what to do about them. Make sure you log enough information to answer the call. Sometimes, you're just better off displaying a message that says, "Please call someone who can figure out what went wrong".
Related
We are Refactoring (and of-course redesigning) our Services in layered design.
We have Service operations layer (BLL), Network abstraction layer -> (deals with network proxy), Data abstraction layer.
But we are a bit baffled about our exception handling strategy.
We don't want to expose too much information from BLL to outside world. (from other layers to bll is fine)
We don't want to clutter the code with try catch stacks
We don't want to mess the exception handling code(like logging, emailing etc) in catch blocks
Could someone post some code samples or literature pointers which we can use to design our simple exception handling framework?
We don't want to expose too much information from BLL to outside world.
(from other layers to bll is fine)
It's BLL itself that defines what's exposed. Make sure You show what's intended to be seen.
We don't want to clutter the code with try catch stacks
Then don't. Exceptions are exceptions. Don't control flow using them. Let them blow up.
We don't want to mess the exception handling code(like logging, emailing etc) in catch blocks
If Your logic does not rely on exception handling (which it should not) and Your code guards itself (this one is important, Your application should ALWAYS blow up on invalid state instead of working further. otherwise - it's hard to understand what causes what), then it's more than enough with wrapping whole app with only 1 error handler that dumps stack trace where necessary.
E.g. - in .net, You can use subscribing to appdomain unhandled exception event for that.
I personally use ELMAH for my web application - few lines in app.config and I have nice error log, stored in sqlite, easily accessable from web app itself. That's about all error handling I got.
Eric Lippert has a wonderful article on how to handle exceptions. I think it would be useful.
Exception handing can be as complex as you want but the good way is to use some global definition. For example by aspects which you can build with any AOP framework - part of most IoC containers like Unity, Windsor Castle, Spring.NET. Separate category of AOP frameworks is PostSharp which adds aspects on compile time insted of runtime.
Also you can check Enterprise Library 5.0 and its Exception handling application block which allows you to do policy based exception handling out of the box.
I have a situation where one of our developers wants to put a standard in place where we throw System.ComponentModel.DataAnnotations.ValidationExceptions in all our apps moving forward. An example would be that a user enters bad data into a form and our business logic layer throws a ValidationException which is handled at the calling layer.
However, I'm worried that this exception class is being used out of context and that one day we'll using some Dynamic Data Controls which make use of this exception and it will be difficult to tell the difference between when he's making use of the ValidationException vs times that the Dynamic Controls raise the exception.
We already use a custom exception class called something like "OurCustomException" and I think it would be better to just subclass off of that and create a OurCustomValidationException class. This way exceptions of different types can be clear cut.
Any opinions?
... it will be difficult to tell the
difference between when he's making
use of the ValidationException vs
times that the Dynamic Controls raise
the exception.
I think this is the main point you should be looking at when making that decision.
You seem to imply that the above (not being able to distinguish your own exceptions from "platform" validation exceptions) is a bad thing. That's not necessarily the case. IF you use the ValidationException exclusively to represent validation errors, then all your code can deal correctly with both your own and platform exceptions in the same way. No need to special-case platform exceptions from custom ones.
This is a win in my opinion. If you have both CustomException and ValidationException going back up to your toplevel layer for the same reasons, you'll have to repeat some logic one way or another. That's a bad thing (more maintenance, more chances of bugs creeping in).
So my opinion is that using the platform ValidationException is probably a good way to do it as long as you use it strictly for propagating validation problems.
Also think about the case where you would be giving/selling parts of your code to a third party (say it's real cool and you make a product out of it). It would probably be easier for the third party if your module throws "standard" exceptions (they can integrate it easily) rather than having to special-case all of his interface code for your module. Again, this is only valid if you stick with cases where a standard module would throw ValidationExceptions.
Lets look at it the other way around. You say:
our business logic layer throws a ValidationException
This is why I put strictly and exclusively above. You need to make sure you agree on what a validation error is. Lets look at two hypothetical problems:
"abc" is not a valid number
insufficient funds for this operation
For 1., the problem is simple/expected input validation error.
But in my opinion, 2. is not. It's a business logic problem. You could call it a validation issue (in the transaction, before the debit, you "validate" whether there are sufficient funds available) but I'd say its semantically very different.
I would advise not to put these two types of errors in the same exception "bag" as they have very different meanings, and may (often) lead to different application flow logic. (With the above two examples, 1. should keep the user on the very same form, as would any other "typo" kind of issue, but 2. should probably get him to a page that allows him to refill his account.)
To sum it up: using a standard exception seems like a good idea to me, as long as you stick with its expected semantics.
I have an application that has many tiers.
as in, i have...
Presentation Layer (PL) - > contains all the html
My Codes Layer (CL) -> has all my code
Entity Layer (EL) -> has all the container entities
Business Logic Layer (BLL) -> has the necessary business logic
Data Logic Layer (DLL) -> any logic against data
Data Access Layer (DAL) -> one that accesses data from the database
Now i want to provide error handling in my DLL since it is responsible for executing statement like ExecureScalar and all....
And i am confused as to how to go about it...i mean do i catch the error in the DLL and throw it back to the BLL and from there throw it back to my code or what....
can any one please help me how do i implement a clean and easy error handling techinque
help you be really appreciated.
Thank you.
You can look at the MS Enterprise Block for error handling and logging. It is nice in terms of configurability. Alternatively Codelpex (codeplex.com) is the community site for MS technology open source projects. Codeplex also have some error handling libraries.
In my opinion it all depends on the kind of exception and what kind of handling you want to with it.
Some errors need to be presented to the user of course. If your application relies heavily on a database connection and your database server is unreachable you need to 'bubble' (not sure if that is the right term?) your event all the way up to your GUI and let the user know that an error occurred.
But if other errors can be handled by your application itself, you just need to 'bubble' the event up to the layer where you can handle it.
As said, there are loads of libraries which can help you handle and log errors easily and the choice for such a library is completely dependent on your project and what suits your needs.
Handle all Excpetions in DLL only, call generateEmail() for notifying admin and send only user defined error messsage from DLL-->BLL-->web-Page and redirect from here to Custom error pages
.on DLL layer generate Email along all exception info with idictionary custom defined arguments (this is what i use for my arguments)
.try catch only in DLL only
.Don't re throw exception on any layer..bad programming techniques
.send error message from DLL--BLL-->web page
.check if is the errfield !="" response.redirect("errorUserPage.")
This title begs for more explanation.
Basically, I'm rolling an API that wraps a web service in a nice heirarchical model. This model exposes things in the form:
var obj = MyRemoteResource.GetForId(1234, SourceEnum.ThatSource);
ApiConsumerMethod(obj.SomeProperty); //SomeProperty is lazily loaded, and often exposes such lazily loaded properties itself
... etc ...
Where many different RemoteResources* (each with many properties exist). There's really aggresive cacheing going on, and request throttling to prevent inadvertantly DOS'ing the servers (and getting the IP of the caller banned).
I've got all this code working, but I'm not doing much in the way of error handling at the moment. Basically, if the consumer of an API provides an invalid ID, the web server is down, a connection times out, or any other of a plethora of request layer errors occur an exception just percolates up when a property is accessed.
I consider this far less than ideal.
So my question is, how should I wrap these errors up so that it is convenient for a user of this API to manage them?
Some routes I have considered:
Just wrap all exceptions in some API defined ones, and document them as thrown.
Expose a static ErrorHandler class that allows a user to register notification callbacks for specific errors; falling back to the above behavior when no registration has been made for specific errors.**
Null properties on error, and set a LastErrorCode.
Each of these approachs have strengths and weaknesses. I'd appreciate opinons on them, as well as alternatives I haven't thought of.
If it impacts the discussion at all, the platform class throwing these exceptions is WebClient. Furthermore, usage of WebClient is sufficiently abstract that it could easily be replaced with some other download scheme if needed.
*Which is to say, many different classes
**This would be... wierd. But it maps to the global nature of the failure. This is my least favorite idea thus far.
I wouldn't implement fancy error technologies (like events and stuff like this). It's not easy to judge where and how to use exceptions, but this is no reason to implements other stuff.
When you request an object by an id which doesn't exist, what do you have to tell the caller about this? If you just return null, the client knows that it doesn't exist, there is nothing more to say.
Exceptions force the caller to care about it. So they should only be used where the caller is expected to do something special. Exception can provide the information why something didn't work. But if it is an "error" the user could also ignore, an exception is not the best choice.
There are two common alternatives to exceptions.
Use return values which provide information about the result of an action. For instance, logon could return a LogonResult instead of throwing an exception.
Write two methods, one throwing an exception, and one (Try...) returning a boolean. The caller decides if it wants to ignore the "error" or not.
Personally I believe this is entirely dependent on the context of what your API end-user is trying to do. If this is the case, you should let them decide how to handle erors.
Recently when working with CSV files (very error prone) I used an API that allowed you to define the exception behaviour. You could either ignore the error, replace by some default value/action, or pass them off to an event handler.
I really liked this approach of letting the end user decide how to handle internal exceptions, because unless your API is very specific its difficult to preempt all scenarios.
I prefer to have an exception thrown at me when something bad happens. Exceptions are (for me, anyways) the common way to tell that something went wrong, and i find it more intuitive to catch, than to check a return value for null and check another property to see what went wrong..
How do I implement error handling elegantly? For example, my data access layer can potentially throw 2 types of errors:
1) not authorized access, in which case the page should hide everything and just show the error message
2) errors that inform the user that something like this already exists in the database (say name not unique - for example), and in this case I wouldn't want to hide everything.
EDITED:
As a result of some comments here I devised that I should create derived specialized exception types, such as NotAuthorizedException, DuplicateException, etc etc.... it's all fine and dandy, however I can see 2 problems potentially:
1) Every stored proc has a return field p_error where it contains an error message. Upon getting the data from DB, I need to check this field to see what type of an error has been returned, so I can throw an appropriate exceptions. So, I still need to store my error types/error messages somewhere.....In other words, how do I should the exact message to the user (at certain times I need to) w/o checking the p_error field first. WHich brings me back to error object. Anyone?
2) I can this potentially turning into a nightmare where the number of exceptions equals the number of error message types.
Am I missing something here?
Much thanks to everyone!
You should check out the exception handling block in Enterprise Library. Lots of good tips and codeware surrounding wrapping exceptions and passing them between layers.
Where's your business layer, and why isn't it checking Authorization and integrity? The DAL is too low level to be checking those rules - if you hit a problem there, it's pretty much time to throw an exception. Your business layer or controllers can catch that exception, and display a reasonable message - but it's not something you should regularly be doing.
Create your own exception layer.
DALExceptionManager
DuplicateException
DatabaseException
BLLExceptionManager
NotAuthorizedException
InvalidDateException
In your Presentation Layer, Add this references and create a common exception handler.
In this way you know how to deal with the exception messages.
One option that I was thinking of
using is create an Error class, but
then I would need to pass it from UI
to business layer and the then to data
access layer by reference
I am not sure I understand this. You don't have to pass the error object in every layer. For example, in one of your example, errors that inform the user that something like this already exists in the database (say name not unique - for example) , a sql exception could be thrown by the framework, and you just need to catch the specific exception in your business layer, or UI layer.
Exception handling block by the Enterprise library suggested by other people will allow you define some policy-based exception handling in your web.config file. It could be good place if you want to develop some enterprise application. But for simple application, you may need not go that far.
What happens in the upper layers isn't up to your Data Access Layer. It shouldn't even be aware of what the upper layers are going to do. If you've got a duplicate key error, then it should throw something like a "DuplicateKeyException". If you should hit an authorization error (I presume you mean "exception"), then don't do anything with it - let it bubble back up to the UI layer, which can display an appropriate error page.
Remember that error status values and such things are the reason we invented exceptions.
Enterprise library exception handling block is the bomb as many have pointed out. Using policies you can do things like logging the exception, wrapping it in a different exception, throwing a new exception instead of the original one. Also, if you are looking to perform specific actions based on authentication errors or duplicate record errors etc, you could always create specific derived exception classes and catch those exception types which would precludes the need of passing any objects from the top down. Exceptions should always bubble up not down.