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So I have been doing some research into how I should be doing try-catch-finally blocks and there is some conflicting information in every post I read. Can someone clarify?
One common idea is to not catch exceptions that you do not know what to do with at that point in the code. The exception will bubble up until it presumably gets to a global exception handler if nothing else catches it. So at that point you display a message to the user that an unknown type of exception occurred, log it, etc.
Now after reading it sounds like this is the only exception handler that you will need? You should not be using it for flow control, so you should be checking if something is returned as null or is invalid causing the exception and correcting it in code. ie. testing for null and doing something about it before it can cause the exception.
But then there are other things such as running out of memory that you would not be able to test for, it would just occur during an operation and the exception would be thrown. But I would not be able to do anything about this either so this would be bubbled up to the global handler.
I think there are some that I am missing, like when dealing with files there can be some thrown from the external code. File not found exception seems like one that may come up often, so I would catch it and in the finally block gracefully close down anything I opened related to other code/processing and then notify the user and log it right there?
The only reason why you would want to catch an exception is for the finally part of the block to make sure that whatever you started before the exception is closed/finalized in a known state? But even then you would want to throw this exception after performing these tasks so the user is notified by the global exception handler, there is no point duplicating this code at this point?
So other than a global exception handler you would have try-catch-finally blocks for these scenarios.
So assuming that I am missing something here, there may be the possibility that you want to try and catch a specific type of exception and then do something with it. I cannot think of anything that you would want to do in the catch block though since the global one would log/notify the user and if you have an exception that usually means that there is no deal for the code to continue on.
Is there any easy way to know which exceptions will be thrown from which modules? I think the only way I have read is to read the MSDN or component suppliers documentation, other than that there is no way to know what exception you would be trying to catch if you were looking for a specific one (not sure why you would)
This question came up since in my application I had a section of code in a try-catch block, and it ended up that when an exception occurred it was either because an object was null, or a string was invalid. Once I wrote code to handle those scenarios the try-catch block is no longer needed, if an exception is encountered now there is nothing the code can do to recover so it should be logged and let the user know so it can be fixed.
But this goes against what I have been reading and what has been preached to me, bad code is code with no try-catch blocks. So how does this all tie together and what piece am I missing here?
The first part of your question is all correct: you should only catch exceptions that you know how to handle. Otherwise, just let them bubble up until they reach code that can handle them.
(Note that "handle" doesn't mean "log" or "display an error". It means to correct the problem that caused the exception, or work around it in some way.)
If they never encounter code that can handle them, or if they are unhandlable exceptions (like OutOfMemory), then they will eventually reach the global unhandled exception handler. This is where you will log the exception (if appropriate), display a generic error to the user (if appropriate), and more often than not, terminate the application. You cannot simply continue as if nothing happened—the exception indicates that the application is in an unexpected state. If you try and continue, you're just going to crash, or worse.
I think there are some that I am missing, like when dealing with files there can be some thrown from the external code. File not found exception seems like one that may come up often, so I would catch it and in the finally block gracefully close down anything I opened related to other code/processing and then notify the user and log it right there?
FileNotFound is a good example of an exception that you will want to handle locally. In the same method (or perhaps one level up, in your UI code) that attempts to load the file, you'll have a catch block for FileNotFound exceptions. If appropriate, display a friendly error message to the user and ask them to choose another file. If it's internal code, give up and try something else. Whatever you need to do. There are few good reasons for FileNotFound to bubble up outside of your code.
This is sort of like using exceptions for flow control, but unavoidable. There is no way to avoid using exceptions (or error codes) for I/O, so you just need to handle the failure case. You could try and verify that the file exists first, before trying to open it, but that would not solve the race issue wherein the file gets deleted or becomes otherwise inaccessible between the time your verification code runs and when you actually try and open it. So now all you've done is duplicated your error-handling code in two places, which serves little purpose.
You have to handle exceptions like FileNotFound locally. The further away from the code that throws, the less likely you can do anything reasonable about it.
Another good example of this, aside from I/O-related exceptions, is a NotSupportedException. For example, if you try to call a method that isn't supported, you might get this exception. You will likely want to handle it and have code in the catch block that falls back to a safe alternative.
The only reason why you would want to catch an exception is for the finally part of the block to make sure that whatever you started before the exception is closed/finalized in a known state? But even then you would want to throw this exception after performing these tasks so the user is notified by the global exception handler, there is no point duplicating this code at this point?
This does not require catching the exception. You can have a try block with only a finally block. A catch block is not required. In fact, this is precisely what using statement implements. If you have state that needs to be cleaned up in the event of an exception being thrown, you should implement the IDisposable pattern and wrap usage of that object in a using block.
Is there any easy way to know which exceptions will be thrown from which modules? I think the only way I have read is to read the MSDN or component suppliers documentation, other than that there is no way to know what exception you would be trying to catch if you were looking for a specific one (not sure why you would)
Precisely. This is not really a problem, though, since you are only catching the exceptions that you can do something about. If you don't know that a module can throw a particular exception, you obviously can't have written code that can handle that exception.
The documentation will tell you all of the important exceptions that you might need to handle, like FileNotFound, SecurityException, or what have you.
This question came up since in my application I had a section of code in a try-catch block, and it ended up that when an exception occurred it was either because an object was null, or a string was invalid. Once I wrote code to handle those scenarios the try-catch block is no longer needed, if an exception is encountered now there is nothing the code can do to recover so it should be logged and let the user know so it can be fixed.
Avoiding exceptions in the first place is always the best option. For example, if you can design your application so that a null object or invalid string is impossible, great. That is what we call robust code. In that case, you don't need to catch these exceptions because there's no way that you can handle it. You thought you already handled the problem, so if an exception is getting thrown anyway, it is a sign of a bug. Don't gloss over it with a catch block.
But sometimes, catch blocks are still necessary, and you write code inside of the catch block to handle the problem. In that case, there's probably no reason to re-throw the exception or log it, so you don't have any code duplication.
But this goes against what I have been reading and what has been preached to me, bad code is code with no try-catch blocks. So how does this all tie together and what piece am I missing here?
Completely wrong. I don't know where you've been reading that, but it is nonsense. Exceptions are exceptional conditions. If your code has catch blocks strewn all over it, that is a sign that you are doing it wrong. Either you're using exceptions for flow control, you're swallowing exceptions in a misguided attempt to "improve reliability", or you don't know about the global unhandled exception handler.
Doesn't sound like you're missing anything to me.
The only thing I feel compelled to mention that doesn't fit strictly into any of your questions is that sometimes you might want to catch an exception and rethrow it as a different exception. The most common situation where you would do this is if you were designing a library of re-usable code. Inside of the library, you might catch internal exceptions and, if you cannot handle them, rethrow them as general exceptions. The whole point of a library is encapsulation, so you shouldn't let exceptions bubble up that the caller cannot possibly do anything about.
There is no true guide for exceptions management (raising and handling). Every app has to decide what level of flow control should be used and how exception has to be raised/handled.
General rules are:
exceptions are raised in exceptional situations
handle exception you can handle and do meaningful things for your app so
exception raising can be used in flow control, actually it's the only way you can reliably handle flow control when you are dealing with devices, so hardware interrupts. (printers, bill validators, file transfer...)
The rest is up to you. The meaning of exception management is made by you.
Imagine you need to download some files from an FTP server, one you don't control. Of course you can't trust other people so you need to prepare for temporary outages and such. So you wrap your downloading code in a try-catch-block and look for WebException if you catch one you might check for FtpStatusCode.ActionNotTakenFileUnavailableOrBusy if that was the error you simply retry. Similarly if you call a web service a 404 might be trouble, but a 429 means that you wait a little and retry, because you had been rate-limited.
Usually you can know which exceptions can be thrown by experience or documentation but C# lacks checked exceptions. Things like NullPointerException or ArgumentNullException can be properly handled with guards in your code. But other things, like errors external dependencies can sometimes be caught and handled by you without crashing the application.
I've been given a task to create a general exception handling code snippet, I have a couple of questions:
Is it a good idea? General exception handling leads to generalized messages as to what's breaking, making tracking hard.Leading to :
What should I include in the snippet? I figure less is more here but adding a log seems to be a good idea because I don't think the exception messages are going to be very specific.
I wouldn't say it's a good idea, no. I tend to have relatively few exception handlers in my code. They're typically there to:
Occasionally convert an exception of one type into another (although that's rarer in C# than in Java; whether it's a good thing or not is a different discussion)
Catch errors at the root of the stack for a particular request / user action / whatever, usually just logging the result
Handle bone-headed APIs which throw exceptions in non-exceptional situations
None of these takes long to write, and none of them comes up so often that it's worth having a common snippet.
Having a set of documented conventions around exception handling - with a discussion of the design in the same document - is a good idea though.
it's better to add specific exception handling and including logs... But, you can add the general exception as well, to catch all unknown exceptions.
Stack traces (to those who can read them) are 90% of what you need. Including the parameters passed into the erroring method will also GREATLY help in debugging. If this is logging to a database, please be careful about logging sensitive pieces of data (PII or PHI).
If this is a web application, I would recommend saving a snapshot of the session as well, this can also help greatly in debugging.
I would recommend you to use Code Contracts and AOP tools like PostSharp. They both provide great possibilities to debug and error-handle your code.
I always use these guidelines as what should be handled by exceptions and what should not. Also, it's hard to get exceptions right.
What I usually end up with is code that has few, if any specific exception handlers, and a global catch-all handler, that just logs the exception (including the stack trace).
That said, when you do use local exception handling, and if the default try.snippet is lacking, something like this might work (just the code included, the rest of the .snippet file is a bit obvious):
try
{
$selected$
}
catch ($caughtExceptionType$Exception ex)
{
$end$
Logger.Error("$message", ex);
//throw new $customExceptionType$Exception("$message", ex);
}
I think that a general exception handler strategy is only applicable at the entry points in the code that you'd like to treat unhandled exceptions. Maybe in a Main method for single-threaded application code or in the AppDomain.UnhandledException event.
Then, the strategy to use is highly application specific. Maybe you have a central log to add the information to, or a message queue that will trigger further processing (e.g., insert in a database and inform an administrator).
First, I'm already familiar with the simple exception handling syntax but I'm asking about the best place, the best time and the best way to deal with them.
I'm building an N-Layered application. so I think the DAL will sometime generate some errors to handle .. and I just learned about the SqlException class, what's the deal with that class ? I once saw a code that handles the SqlException then it handles Exception!
After knowing the practice and where I'm going to handle them, I'm planning to create a method to connect to the database and log the errors in a database so I could fix it but still I don't know what information should I collect to allow me identify the whole situation!
I thought exceptions handling was not a big deal. but every now and then I read some strange advices -that I never understood- on the questions comments but no one could answer me since it was some very old questions!
"Don't just explicitly catch
exceptions"
"the code that is used by
higher-layers in your application must
always only throw exceptions and never
worry about how to deal with them."
EDIT
What about Page_Error event and Application_Error .. I saw that they are a good practice for handling errors
Exception handling is a big deal, and it's not simple to design a good strategy for that.
First of all, some general rules:
Exceptions occur when the running code is completely unable to go ahead, so maybe it tried to handle some internal exceptions but ultimately failed. Think about TCP connection: if a damaged packet arrives, it's an exception, but TCP protocol can handle it. If too many are damaged, an I/O or socket exception is thrown
Exceptions can not always be handled. In almost all cases, when you get an exception from underlying layers you are unable to run corrective code. If your application depends on a DB and that is offline, when you get the exception about it you can only display an error message
Exceptions can be unexpected, and can reveal design or implementation flaws. For example, an implementation flaw can be the situation in which you have a redundant DB but when you fail to connect to frist mirror you don't try with the second
For the third point, it's important to log exceptions and periodically analyse logs to find any weird situation. So, let's begin with the concrete answer.
First of all
think about "handling" the exception. When you write every single code line, think about the possible problems that may prevent it from completing, and think about the possible corrective actions. if any are possible. An error message is not a good handling way, it's the latest strategy.
Don't start to write try-catch(Exception), but prefer specific exceptions. If you need to parse strings to numbers etc, then expect FormatException, if you need to cast from Object to your type expect InvalidCastException
When you write lower-level layers
don't hesitate to throw exceptions!! Don't do like many folks do, ie. return null or use (like ANSI C) a boolean return value and reference parameters. Exceptions are there for that. If you can handle an exception (ie. you don't find a local file but you know you have a remote backup, so handle FileNotFoundException by calling the remote mirror, but if you can't still connect then ultimately throw) then do it and try to resume computation, but if you cannot then throw. And don't forget to throw the inner exception, if present, because it is helpful for logging in the highest layer.
Basically, you can still decide to throw an exception on your own even if you don't catch any! And this is highly recommended especially when function parameters are invalid!
Another good option is to still log in the underlying layers. You actually want to log no matter an exception occurs.
When you log
remember to give an adequate severity to the messages. If you find via code that your DB is offline, that's not an unexpected exception. Still log it as an error, but don't worry about code bugs when you investigate the logs. Instead, if you catch an exception that your code is unable to recognize (a NullReferenceException is a classic example) then log with highest severity, ie. fatal, to give it maximum priority!
A good strategy for ASP.NET
can surely be based upon Page.OnError method. If you have a base page class for all of the pages of your site, you should definitely override that method. In that method, you should first log your exception.
You also shouldn't abuse of try-catch(Exception) blocks, because if you don't catch an exception you can't handle with catch, you will have to handle it via OnError.
When you run such a method, don't immediately think about Server.RemoveError(). You can prefer to have a static HTML page for HTTP 500 error (that is triggered when an unhandled exception bubbles to ASP.NET runtime) that displays a courtesy message to the user.
Briefly
Don't hesitate to throw in underlying layers if anything strange occurs
As said by your advice, don't handle exceptions you are unable to handle (if you catch an exception you can't handle, rethrow it)
LOG!!!!!!!!!!!!!!!!!
Don't disclose exception details to final users on a public website, never!! By default, ASP.NET prevents that from occurring, but you could still use OnError to print stack trace
Use OnError, or Application_Error as single central point to handle all unexpected exceptions
Periodically examine logs against error/fatal messages to find issues with your code, then think about maintaining/debugging/fixing it
Take a look at elmah. It's a logger for asp.net. Renders all errors on a nice summary page.
http://code.google.com/p/elmah/
The best way to handle exceptions is in the specific layer they apply to. If it is a constraint volation, for example, 2 users with the same name, you should let that bubble up to the UI and alert the user.
Same goes with any business rule violations. Those should bubble up to the UI so the end user knows what went wrong.
A SQL Connectivity error is best handled in the DAL...etc..
The how/when/where to catch exceptions may depend on what your trying to do exactly, its difficult to give an exact catch all always correct answer.
As to your specific questions,
I just learned about the SqlException
class, what's the deal with that class
? I once saw a code that handles the
SqlException then it handles
Exception!
Its good practice to handle the specific exception you believe may occur, if your not sure what type this exception is you can just 'Exception', if you want something specific to occur on a 'SQLException' and something else to happen with an 'Exception' then there is certainly nothing wrong with writing code that handles both.
"Don't just explicitly catch
exceptions"
I believe this is refering to code like this
try
{
int i = 1/0;
}
catch(Exception e)
{
//do nothing
}
This exception will be caught but you'll never know it happened, hence this is not a good idea, and the person using the code will be scratching their head as to whats going on.
I think what you are asking here is a Error/Exception Handling Strategy for any application.
I think it includes:
Where - All places where you think an exception can occur or which need more monitoring like DB calls, External Service Calls, Use of Arrays, User Input Parsing, Type Casting and so on...
How - All you high level layers should throw the exception and it should be captured at the entry point and processed to understand the root cause. Usually you do this in Application_Error() where you catch the exception and log it for troubleshooting. How you log an exception is upto you. A Log File or DB driven log is an option based on your requirements and available resources.
IMO apart from extremely rare circumstances I only ever use exception handling for I/O related code where there are interactions with services and file systems whose functionality and maintenance is beyond the control of my applications.
I have always considered the use try/catch statements to manipulate the logic (flow-of-control) in a program in the same way if/else statement work to be extremely bad practice. Most common exceptions can be avoided if you use the tools at hand correctly.
I've found myself doing too much error handling with try\catch statements and getting my code ugly with that. You guys have any technique or framework to make this more elegant? (In c# windows forms or asp.net).
You need to read up on structured exception handling. If you're using as many exception handlers as it sounds then you're doing it wrong.
Exception handling isn't like checking return values. You are supposed to handle some exceptions in limited, key spots in your code not all over the place. Remeber that exceptions "bubble up" the call stack!
Here is a good and well-reviewed CodeProject article on exception best practices.
Java land had pretty the same problem. You just look at method and you can't at a first glance understand what it is doing, because all you see is try/catch blocks. Take a 30-40 line method and throw away all try statements and catch blocks and you might end up with 5-6 lines of pure application logic. This isn't such a big problem with C# as it has unchecked exceptions, but it gets really ugly in Java code. The funny thing is the try/catch blocks were intended to solve the very same problem in the first place. Back then it was caused by errno/errstr madness.
What the Java guys usually do is based on how do you typically handle exception. Most of the time you can't really do anything to correct the problem. You just notify the user that whatever he was trying to do didn't work, put back application in a certain state and maybe log and exception with complete stacktrace to log file.
Since you handle all the exceptions like this, the solution is to have a catch-all exception handler, which sits on top of application stack and catches all exceptions that are thrown and propagated up the stack. With ASP.NET you might use something like this:
http://www.developer.com/net/asp/article.php/961301/Global-Exception-Handling-with-ASPNET.htm
At the same time you are free to override that global handler by placing try/catch block in your code, where you feel something can be done, to correct the problem.
Just adding a Try Catch does not solve the problem. This topic a too big to handle as one question. You need to do some reading.
http://msdn.microsoft.com/en-us/library/8ey5ey87%28VS.71%29.aspx
http://www.codeproject.com/KB/architecture/exceptionbestpractices.aspx
Also FXCop, and VS Team System will warn you on some design issues.
Such heavy reliance on exception handling (in any language) does suggest that the mechanism is being misused. I always understood that exception handling was designed to trap the truly exceptional, unforeseeable event. It is not designed to handle (for instance) invalid data entry by a user - this is normal operation and your design and application coding must deal with such normal processing.
http://msdn.microsoft.com/en-us/library/ff664698(v=PandP.50).aspx
Check out Microsoft's Exception Handling Application Block. It has an intial learning curve, but is good stuff once you get it figured out.
I'm build an UserControl and I'm not sure how to handle exceptions, the Control itself is not very complicated, the User chose an image from disk so they can authorize it, I don't know exactly how the control will be used so if I use a MessageBox I might block the application, and if I just re-throw it I might crash it.
thanks in advance.
Juan Zamudio
this is a common problem facing developers who build libraries. Try to weed out bugs and decide for the remaining error cases if it's an expected error (your control should not throw an exception but rather gracefully handle the error) or an unexpected exceptional condition (your control must throw an exception as soon as possible).
You might also have a look at Design By Contract, a methodology to declare required preconditions and guaranteed postconditions. This may sound academic, but it leads to more robust code.
UPDATE:
A good introduction is http://se.ethz.ch/~meyer/publications/computer/contract.pdf
Regards,
tamberg
unhandled exceptions should definitely be thrown so that the people using your control can see what's wrong.
Only handle exceptions that you know of and know what to do with. Don't bother with generic handlers, like a MessageBox. Just let it propogate to the application where there's more context for diagnosing the error. It's their responsibility to catch any exceptions so that the application doesn't crash. In the case of an exception they can't do anything about because it has to do with the control's internals, you should handle that yourself and if it's partially the user's fault, wrap the exception with a message saying what is missing, with the original exception available via the InnerException property.
In addition to what's been said, I also want to mention that you should try to have your control avoid exceptions by checking for different object states and "preventing" rather then allowing an exception to be raised.
Keep mind mind that throwing an exception is a rather expensive process and (as I've been told in the past) exceptions should be reserved for truly "exceptional" unexpected cases...
Best Regards,
Frank