Related
Just a thing I've been thinking of for a while. Do I need to handle KeyNotFoundException by catching that specific exception or can I just use a "blank" catch like this:
try
{
//Code goes here
}
catch
{
}
Or do I have to do it like this:
try
{
//Code goes here
}
catch(Exception ex)
{
}
Or do I have to do it like this:
try
{
//Code goes here
}
catch(KeyNotFoundException ex)
{
}
The reason why I ask is that when I look at crash count at App Hub I have a lot of crashes related to "KeyNotFoundException" but I never experience any crashes in my app. Could this be the problem, that I don't catch the specific exception and that App Hub crash statistics classifies it as a crash even if the exception is handled?
EDIT:
Here are some screenshots of the App Hub crash statistics (Stack Trace). Does anyone know ehat in detail it means? It has to do with my background agent and that might be the reason for why I never experience any crashes in my app:
No, the marketplace is counting only unhandled exceptions, so your app does crash.
An empty catch or catching Exceptions are the most general catches (Every exception is derived from the Exception base class, so you're catching everything.), the critical code is somewhere you don't use try-catch. Based on the exception you should check your dictionaries and think about what are the conditions which can cause error.
Generally a good practice is to check the correctness of parameters in your public methods so if any problem occurs you can provide yourself more helpful error messages, for example:
public User GetUser(string username)
{
if (String.IsNullOrEmpty(username))
throw ArgumentNullException("username");
return this.users[username];
}
In this case if things goes wrong you will see that you used a null for username, otherwise you would see a KeyNotFoundException. Hope this helps, good luck!
You can use a base exception to catch a more derived exception, so Exception will catch KeyNotFoundException because the latter inherits the former. So strictly speaking, if you want to catch "any" exception, catch (Exception) will suffice.
However, you should only catch exceptions if you can handle them in some meaningful manner. Though I'm not sure how this mindset stacks up against WP development.
As for your underlying problem, I've no idea. Does the App Hub not provide any details around the crash such as stack traces?
Your best bet is to leave the template code in place that registers for the unhandled exceptions event and put some logging into your application to record as much detail as you can about the state of the app during the crash.
No, you do not have to catch each specific exception type in a try / catch block, see the C# language reference.
However, rather than wrapping all your code in try / catch blocks, you probably want to add exception handling logic and logging into a handler for the Application.UnhandledException event. See this excellent blog post for an example of how to handle this event.
If you are interested in a specific exception, such as KeyNotFoundException in a particular part of the code then you catch it like this
try
{
//Code goes here
}
catch(KeyNotFoundException ex)
{
}
If you want to catch a specific exception and some undefined one you do something like this
try
{
//Code goes here
}
catch(KeyNotFoundException ex)
{
}
catch(Exception ex)
{
}
If you want to make sure your application doesn't crash use Collin's example with the Application.UnhandledException event.
You can catch all exceptions by catching the base class, but whether you want to depends on what your trying to achieve.
Generally speaking, you should only catch an exception at a level at which you have the knowledge to decide what should be done about the error, ie roll back some action, or display a message to the user. It is often the case that at a certain level it makes sense to catch a specific exception type, as that level of code understands what that means, but it may not make sense to catch all.
Avoid catching everything too soon, exceptions exist to tell you somethings wrong, blanket catching ignores that and can mean your program keeps running, but starts behaving wrong, possibly corrupting data. Its often better to "fail early and fail fast" when receiving unexpected exceptions.
As others have said - No - you dont need to catch the specific exception, catching Exception or just catch will prevent the exception from bubbling up.
However you should just catch specific exceptions where possible to make your code more explicit in what it is doing. Better again test for correctness before the potential error condition - again this is covered in other posts.
For your specific problem the link you posted seems to indicate that it is a problem with reading values from the isolated storage (IsolatedStorage.get_Item) - so wherever you access IsolatedStorage during from the ScheduledTaskAgent invocation you need to ensure the item exists before getting it. Perhaps there are some config settings missing or something?
This question already has answers here:
Closed 11 years ago.
Possible Duplicate:
Catching specific vs. generic exceptions in c#
Here's an example method
private static void outputDictionaryContentsByDescending(Dictionary<string, int> list)
{
try
{
//Outputs entire list in descending order
foreach (KeyValuePair<string, int> pair in list.OrderByDescending(key => key.Value))
{
Console.WriteLine("{0}, {1}", pair.Key, pair.Value);
}
}
catch (Exception e)
{
MessageBox.Show(e.Message, "Error detected", MessageBoxButtons.OK, MessageBoxIcon.Error);
}
}
I would like to know what exception clause to use apart from just Exception and if there is an advantage in using more specific catch clauses.
Edit: O.k thanks everyone
Catching individual types of Exceptions in your statement will allow you to handle each in a different way.
A blanket rule for Exception may be useful for logging and rethrowing Exceptions, but isn't the best for actually handling Exceptions that you may be able to recover from.
try
{
// open a file specified by the user
}
catch(FileNotFoundException ex)
{
// notify user and re-prompt for file
}
catch(UnauthorizedAccessException ex)
{
// inform user they don't have access, either re-prompt or close dialog
}
catch(Exception ex)
{
Logger.LogException(ex);
throw;
}
You should only really catch exceptions that you are expecting that code may throw. That way, if it throws something you didn't expect, it may either be something critical; something that should bubble up the call stack and possibly crash the application; or something you have not thought of.
For example, you may wish to handle IOExceptions thrown by I/O code so that you can relay the problem back to the user. However, the same operations may throw something more critical such as an AccessViolationException. In this case, you might want the program to terminate, or handle the problem in a different way.
Generic exception handling should only really be used in cases where you do not care what error occurred, and subsequently don't want it affecting the rest of your process.
The only potential cause for an exception that I see in your example is if list is null. OrderByDescending() should return an empty IEnumerable<> rather than a null reference.
If I read that correctly, it might make more sense to catch NullReferenceException:
try
{
...
} catch (NullReferenceException exception)
{
MessageBox.Show(...);
}
However, this really depends on the needs of your application. If your intention is just to alert the user or to log all exceptions, catching the Exception class is fine. If you need special handling for different types of exceptions - such as sending an email alert instead of just logging the message - then it makes sense to use specific exception types:
try
{
}
catch(NullReferenceException e)
{
//...
}
catch(StackOverflowException e)
{
//...
}
catch(Exception e)
{
/// generic exception handler
}
Which exception to use really depends on the code in the try block. In general you want to catch exceptions that you can do something with and let exceptions you have no power over move to high levels of your code where you can perform some action that makes since. One of the most common mistakes I see people make is attempting to catch errors that they have no ability to handle.
for example
Void ChoseFile()
{
try
{
string FileName = GetInputFile()
}
catch( InvalidFileSelectedException ex)
{
//In this case we have some application specific exception
//There may be a business logic failure we have some ability
//to infomr the user or do an action that makes sense
}
catch(FileNotFoundException exfilenot found)
{
//In this case we can do somthing different the the above
}
catch(Exception )
{
//Normal I would not use this case we have an error we don't know what to do
//with. We may not have a usefull action. At best we can log this exception
// and rethrow it to a higher level of the application that can inform the user
// fail the attempted action. Catching here would only suppress the failure.
}
}
You should always catch exceptions with an as specific class as possible.
If you know what to do if a file is locked, but you regard all other errors as unexpected and impossible to handle, you should catch System.IO.IOException and deal with the error. You should only catch Exception (or just catch {) for gracefully exiting.
Since you are dealing with a Dictionary.. then you want to look at these 2 exceptions
The key of keyValuePair is a null reference (Nothing in Visual Basic).
ArgumentException An element with the same key already exists in the Dictionary(TKey, TValue).
KekValuePair Exception
This is taken from the MSDN site
Use the exception type that you might expect but still not be able to prevent and that you can adequately handle. Let anything else bubble up to somewhere that might expect it or can handle it.
In your case here, I might expect that I would run into a NullReferenceException if the dictionary is null. But I would not catch it. This is something I can validate against instead
if (dictionary != null)
So there is no reason to allow an exception to even happen. Never use exceptions for control flow, and validate against known causes.
Some classes/methods will throw different exceptions, depending on the error. For example, you might be using the File class to write data to a file. You could write multiple Catch statements for the exception types you could recover from, and a generic Exception catch to log and bubble up anything that can't be recovered from.
By using Exception you catch all exceptions. Of you use IOException or StackOverflowException you'll only catch errors of that type.
a StackOverflowException catched by a Exception still hold the same message, stack trace etc.
Exception handling philosophy
I am sure you can find many other philosophies
Code defensively. Catching exceptions is more expensive than preventing the error in the first place.
Don't catch an exception and bury it by not handling it. You can spend many hours trying to find an error that has been suppressed.
Do log errors that you catch.
This helps in analyzing the problem. You can check to see if more than one user is having the same problem
I prefer a database for logging, but a flat file, or the event log are also suitable.
The database solution is easiest to analyze but may introduce additional errors.
If the error is due to bad data entered by the user, inform the user of the problem and allow them to retry.
Always allow an escape route if they cannot fix the problem.
Catch the error as close to the source as possible
This could be a database procedure, a method in a data access layer (DAL) or some other location.
Handling the exception is different than catching it. You may need to rethrow the exception so that it can be handled higher up the stack or in the UI.
Rethrowing the exception can be done in at least two ways.
throw by itself does not alter the stack.
throw ex does alter or add to the stack with no benefit.
Sometimes it is best not to catch an exception, but rather let it bubble up.
If you are writing services (web or windows) that do not have a user interface (UI) then you should always log the error.
Again, this is so that someone can analyze the log or database file to determine what is happening.
You always want someone to know that an error has occurred.
Having a lot of catch statements for a try block can make your code more difficult to maintain, especially if the logic in your catch blocks is complex.
Instead, code defensively.
Remember that you can have try catch blocks within catch blocks.
Also, don't forget to use the finally block where appropriate.
For example, closing database connections, or file handles, etc.
HTH
Harv
i know this could be a little weird but a doubt is a doubt afterall...
what would happen in the following situation...
private void SendMail()
{
try
{
//i try to send a mail and it throws an exception
}
catch(Exception ex)
{
//so i will handle that exception over here
//and since an exception occurred while sending a mail
//i will log an event with the eventlog
//All i want to know is what if an exception occurs here
//while writing the error log, how should i handle it??
}
}
Thank you.
I would personally wrap the call to write to event log with another try\catch statement.
However, ultimately it depends on what your specification is. If it is critical to the system that the failure is written to the event log then you should allow it to be thrown. However, based on your example, I doubt this is what you want to do.
You can simply catch errors in the error logging method. However I wouldn't personally do that, as broken error logging is a sign your application can't function at all.
private void SendMail()
{
try
{
//i try to send a mail and it throws an exception
}
catch(Exception ex)
{
WriteToLog();
}
}
private void WriteToLog()
{
try
{
// Write to the Log
}
catch(Exception ex)
{
// Error Will Robinson
// You should probably make this error catching specialized instead of pokeman error handling
}
}
Each exception is caught only when inside a try-catch block. You could nest try-catch but is generally not a good idea.
You could add a try-catch block in your catch block as well.
Considering the kind of exceptions when writing to a file (rights, disk space...) I would advice not to handle it in here. If it fails the first time, there's good chance you won't be able to write to the event log that it's not possible to write in the event log...
Let it bubble up and be handled by an upper level try/catch.
Chris S. has the best answer. Placing a try-catch block inside a catch block is very rarely a good idea. and in your case it will just convolute your code. If you check to see if you were successful in writing to your log file here, you will have to do it in every place where you try to write into your log file. You can easily avoid this unnecessary code duplication by having all your individual modules be self contained when it comes to notifying/handling of error conditions within these modules. When sending your mail fails you perform the proper actions inside your catch block to handle this exceptional condition like:
disposing of the contents of your mail object
making sure your socket is closed
writing an entry into your log file to note the error
Inside your catch block just call whatever API you have defined to writing a log entry into your logfile and forget about about the rest. Inside your logging API is where you should handle any logging related exceptional cases (the disk is full, no permission to write to file, file not found, etc...). Your mailing module does not need to know if the logging was successful or not, that responsibility should be delegated to the logging module.
I personally handle this situation using a simple extension method.
public static class MyExtentions
{
public static void LogToErrorFile(this Exception exception)
{
try
{
System.IO.File.AppendAllText(System.IO.Path.Combine(Application.StartupPath, "error_log.txt"),
String.Format("{0}\tProgram Error: {1}\n", DateTime.Now, exception.ToString()));
}
catch
{
// Handle however you wish
}
}
}
The usage is simple:
try
{
...
}
catch(Exception ex)
{
ex.LogToErrorFile();
}
You can then handle the caught exception inside the extension method however you want, or simply don't catch it and let it bubble up to the top. I've found this design to be a simple, reproducible way to handle exceptions throughout the application.
Firstly I would say don't catch "Exception" in catch block. You could instead, for mailing, check for all validity and then catch specific exception(SmtpException, ) that you can do something about(and informing user with a friendly message). Throwing exception from your code and informing the UI about is not a bad idea. If your methods accepts inputs with certain specification and if they are not met, your method should/can throw error and inform user about it.
For exceptions that have no control over, use global handling exception, like Application_Error for web.
Getting Better Information on Unhandled Exceptions Peter Bromberg explains this better.
Also for any privildged resource you are accessing, like eventlogs, make sure you assembly has access to it.
Useful links Build a Really Useful ASP.NET Exception Engine By Peter A. Bromberg
and
Documenting Exceptional Developers By Peter A. Bromberg
For web application look into
Health monitoring
Exception logging
One more thing, if your application goes wrong/ throws error that can't handle( at all) its better to let it go down gracefully and not continue. Application in unstable state is not good idea.
Disclaimer, I'm from a Java background. I don't do much C#. There's a great deal of transfer between the two worlds, but of course there are differences and one is in the way Exceptions tend to be thought about.
I recently answered a C# question suggesting that under some circstances it's reasonable to do this:
try {
some work
} catch (Exeption e) {
commonExceptionHandler();
}
(The reasons why are immaterial). I got a response that I don't quite understand:
until .NET 4.0, it's very bad to catch
Exception. It means you catch various
low-level fatal errors and so disguise
bugs. It also means that in the event
of some kind of corruption that
triggers such an exception, any open
finally blocks on the stack will be
executed, so even if the
callExceptionReporter fuunction tries
to log and quit, it may not even get
to that point (the finally blocks may
throw again, or cause more corruption,
or delete something important from the
disk or database).
May I'm more confused than I realise, but I don't agree with some of that. Please would other folks comment.
I understand that there are many low level Exceptions we don't want to swallow. My commonExceptionHandler() function could reasonably rethrow those. This seems consistent with this answer to a related question. Which does say "Depending on your context it can be acceptable to use catch(...), providing the exception is re-thrown." So I conclude using catch (Exception ) is not always evil, silently swallowing certain exceptions is.
The phrase "Until .NET 4 it is very bad to Catch Exception" What changes in .NET 4? IS this a reference to AggregateException, which may give us some new things to do with exceptions we catch, but I don't think changes the fundamental "don't swallow" rule.
The next phrase really bothers be. Can this be right?
It also means that in the event
of some kind of corruption that
triggers such an exception, any open
finally blocks on the stack will be
executed (the finally blocks may
throw again, or cause more corruption,
or delete something important from the
disk or database)
My understanding is that if some low level code had
lowLevelMethod() {
try {
lowestLevelMethod();
} finally {
some really important stuff
}
}
and in my code I call lowLevel();
try {
lowLevel()
} catch (Exception e) {
exception handling and maybe rethrowing
}
Whether or not I catch Exception this has no effect whatever on the excution of the finally block. By the time we leave lowLevelMethod() the finally has already run. If the finally is going to do any of the bad things, such as corrupt my disk, then it will do so. My catching the Exception made no difference. If It reaches my Exception block I need to do the right thing, but I can't be the cause of dmis-executing finallys
For the question #2:
The author meant "Corrupted State Exceptions". They will be introduced in .NET 4.0 (CLR team announced this at PDC 2008 in this talk).
Corrupted state exceptions cannot be caught by normal catch block. Examples: Access violation, Invalid Memory.
But you might want to catch these exceptions:
In main() – write to log, exit, turn off addin
Very rare cases when you know that code throws exception like this (e.g. some cases with interop with native code)
To do this you should put attribute [HandleProcessCorruptedStateException] at the method where you want to catch CorruptedStateException.
To read more about these exceptions please see this MSDN article.
As a general rule you shouldn't catch exceptions unless:
You have a specific exception that you can handle and do something about. However in this case you should always check whether you shouldn't be trying to account for and avoid the exception in the first place.
You are at the top level of an application (for instance the UI) and do not want the default behaviour to be presented to the user. For instance you might want an error dialog with a "please send us your logs" style message.
You re-throw the exception after dealing with it somehow, for instance if you roll back a DB transaction.
Your finally blocks always execute. Your code sample is correct - the low level method should just have try and finally. For instance an unmanaged call might know that it needs to dispose of the unmanaged resource, but this wouldn't be exposed to .Net methods calling it. That call should get rid of the unmanaged resource in its finally block, and calling managed methods can handle exceptions or just pass them on up.
If there's something in an exception that you need to handle you should re-throw the exception, for instance:
try {
conn.BeginTransaction();
//do stuff
conn.CommitTransaction();
}
catch (Exception) {
conn.RollbackTransaction(); //required action on any exception
throw; //re-throw, don't wrap a new ex to keep the stack trace
}
finally {
conn.Dispose(); //always dispose of the resource
}
My motto is handle what you can (or need to) and let any other exceptions bubble up and catch them in an UnhandledException event.
You are correct tho, a finally block is always called (even in the event of an exception being raised in the try section) before you exit the method. So whether you want to the catch the exception on the out method or not is entirely up to you.....this should not affect the finally block being called.
Just to your third question:
If you have
nastyLowLevel() {
doSomethingWhichMayCorruptSomethingAndThrowsException();
}
MyEvilCatcher() {
try {
nastyLowLevel();
} catch (Exception e) {
MyExceptionHandler(e);
}
}
WiseCatcher() {
try {
MyEvilCatcher();
} catch (LowLevelException e) {
DoSomethingWiseSoFinnalyDontRuinAnything();
} finally {
DoSomethingWhichAssumesLowLevelWentOk();
}
}
I think the response you asked about just meant that some low level methods could use exceptions to inform some outer method that special care has to be taken. If you forget about this exceptions in your catch-all-handler and don't rethrow them, something might go wrong.
Generally, I prefer thinking carefully about which exceptions can possibly be thrown and catch them explicitely. I only use "generic handler" at the very outer level in production environments in order to log unexpected exception and present them to the customer in an well-formatted manner (including a hint to send us the log file.)
I think the quoted response is wrong (or maybe meant 2.0 instead of 4.0)? It sounds kinda bogus to me.
I am currently doing a code review and the following code made me jump. I see multiple issues with this code. Do you agree with me? If so, how do I explain to my colleague that this is wrong (stubborn type...)?
Catch a generic exception (Exception ex)
The use of "if (ex is something)" instead of having another catch block
We eat SoapException, HttpException and WebException. But if the Web Service failed, there not much to do.
Code:
try
{
// Call to a WebService
}
catch (Exception ex)
{
if (ex is SoapException || ex is HttpException || ex is WebException)
{
// Log Error and eat it.
}
else
{
throw;
}
}
The mantra is:
You should only catch exceptions if
you can properly handle them
Thus:
You should not catch general
exceptions.
In your case, yes, you should just catch those exceptions and do something helpful (probably not just eat them--you could throw after you log them).
Your coder is using throw (not throw ex) which is good.
This is how you can catch multiple, specific exceptions:
try
{
// Call to a WebService
}
catch (SoapException ex)
{
// Log Error and eat it
}
catch (HttpException ex)
{
// Log Error and eat it
}
catch (WebException ex)
{
// Log Error and eat it
}
This is pretty much equivalent to what your code does. Your dev probably did it that way to avoid duplicating the "log error and eat it" blocks.
I am currently doing a code review and the following code made me jump. I see multiple issues with this code. Do you agree with me?
Not totally, see below.
Catch a generic exception (Exception ex)
In general, catching a generic exception is actually ok as long as you rethrow it (with throw;) when you come to the conclusion that you can't handle it. The code does that, so no immediate problem here.
The use of "if (ex is something)" instead of having another catch block
The net effect of the catch block is that only SoapException, HttpException, etc. are actually handled and all other exceptions are propagated up the call stack. I guess functionality-wise this is what the code is supposed to do, so there's no problem here either.
However, from a aesthetics & readability POV I would prefer multiple catch blocks to the "if (ex is SoapException || ..)". Once you refactor the common handling code into a method, the multiple catch blocks are only slightly more typing and are easier to read for most developers. Also, the final throw is easily overlooked.
We eat SoapException, HttpException and WebException. But if the Web Service failed, there not much to do.
Here possibly lurks the biggest problem of the code, but it's hard to give advice without knowing more about the nature of the application. If the web service call is doing something that you depend on later then it's probably wrong to just log & eat the exceptions. Typically, you let the exception propagate to the caller (usually after wrapping it into e.g. a XyzWebServiceDownException), maybe even after retrying the webservice call a few times (to be more robust when there are spurious network issues).
The problem with catching and re-throwing the same exception is that, although .NET does its best to keep the stack trace intact, it always ends up getting modified which can make tracking down where the exception actually came from more difficult (e.g. the exception line number will likely appear to be the line of the re-throw statement rather than the line where the exception was originally raised). There's a whole load more information about the difference between catch/rethrow, filtering, and not catching here.
When there is duplicate logic like this, what you really need is an exception filter so you only catch the exception types you're interested in. VB.NET has this functionality, but unfortunately C# doesn't. A hypothetical syntax might look like:
try
{
// Call to a WebService
}
catch (Exception ex) if (ex is SoapException || ex is HttpException || /* etc. */)
{
// Log Error and eat it
}
As you can't do this though, what I tend to do instead is use a lambda expression for the common code (you could use a delegate in C# 2.0), e.g.
Action<Exception> logAndEat = ex =>
{
// Log Error and eat it
};
try
{
// Call to a WebService
}
catch (SoapException ex)
{
logAndEat(ex);
}
catch (HttpException ex)
{
logAndEat(ex);
}
catch (WebException ex)
{
logAndEat(ex);
}
I would like to add here, because the Exception handling in almost all java / C# code that I have seen is just incorrect. I.e. you end up with very difficult to debug error for ignored Exceptions, or, equally bad, you get an obscure exception which tells you nothing, because blindly following the "catching(Exception) is bad" and things are just worse.
First, understand that an exception is a way to facilitate the returning of error information across code layers. Now, mistake 1: a layer is not just a stack frame, a layer is code which has a well defined responsibility. If you just coded interfaces and impls just because, well you have better things to fix.
If the layers are well designed and have specific responsibilities, then the information of the error has different meaning as it bubbles up. <-this is the key on what to do, there is no universal rule.
So, this means that when an Exception occurs you have 2 options, but you need to understand where in the layer you are:
A) If you are in the middle of a layer, and you are just an internal, normally private, helper function and something goes bad: DONT WORRY, let the caller receive the exception. Its perfectly OK because you have no business context and
1) You are not ignoring the error and
2) The caller is part of your layer and should have known this can happen, but you might not now the context to handle it down below.
or ...
B) You are the top boundary of the layer, the facade to the internals. Then if you get an exception the default shall be to CATCH ALL and stop any specific exceptions from crossing to the upper layer which will make no sense to the caller, or even worse, you might change and the caller will have a dependency to an implementation detail and both will break.
The strength of an application is the decoupling level between the layers. Here you will stop everything as a general rule and rethrow the error with a generic exception translating the information to a more meaningful error for the upper layer.
RULE: All entry points to a layer shall be protected with CATCH ALL and all errors translated or handled. Now this 'handeled' happens only 1% of the time, mostly you just need (or can) return the error in a correct abstraction.
No I am sure this is very difficult to understand. real Example ->
I have a package that runs some simulations. These simulations are in text scripts. there is a package that compiles these scripts and there is a generic utils package that just reads text files and, of course, the base java RTL. The UML dependency is->
Simulator->Compiler->utilsTextLoader->Java File
1) If something breaks in the utils loader inside one private and I get a FileNotFound, Permissions or what ever, well just let it pass. There is nothing else you can do.
2) At the boundary, in the utilsTextLoader function initially called you will follow the above rule and CATCH_ALL. The compiler does not care on what happen, it just needs to now whether the file was loaded or not. So in the catch, re throw a new exception and translate the FileNotFound or whatever to "Could not read file XXXX".
3) The compiler will now know that the source was not loaded. Thats ALL it needs to know. So if I later I change utilsTestLoader to load from network the compiler will not change. If you let go FileNotFound and later change you will impact compiler for nothing.
4) The cycle repeats: The actual function that called the lower layer for the file will do nothing upon getting the exception. So it lets it go up.
5) As the exception gets to the layer between simulator and compiler the compiler again CATCHES_ALL, hiding any detail and just throws a more specific error: "Could not compile script XXX"
6) Finally repeat the cycle one more time, the simulator function that called the compiler just lets go.
7) The finally boundary is to the user. The user is a LAYER and all applies. The main has a try that catches_ALL and finally just makes a nice dialog box or page and "throws" a translated error to the user.
So the user sees.
Simulator: Fatal error could not start simulator
-Compiler: Could not compile script FOO1
--TextLoader: Could not read file foo1.scp
---trl: FileNotFound
Compare to:
a) Compiler: NullPointer Exception <-common case and a lost night debugging a file name typo
b) Loader: File not found <- Did I mention that loader loads hundreds of scripts ??
or
c) Nothing happens because all was ignored!!!
Of course this assumes that on every rethrow you didn't forget to set the cause exception.
Well my 2cts. This simple rules have saved my life many times...
-Ale
Sometimes that is to only way to handle "catch every exception" scenarios, without actually catching every exception.
I think sometimes, say, lowlevel framework / runtime code needs to make sure that no exception is ever escaping. Unfortunately, there is also no way the framework code can know which exceptions are raised by the code executed by the thread.
In that case a possible catch block could look like this:
try
{
// User code called here
}
catch (Exception ex)
{
if (ExceptionIsFatal(ex))
throw;
Log(ex);
}
There are three important points here, however:
This isn't something for every situation. In code reviews we are very picky about places where this is actually neccessary and thus allowed.
The ExceptionIsFatal() method assures that we don't eat exceptions which should never be swallowed (ExecutionEngineException, OutOfMemoryException, ThreadAbortException, etc.)
What is swallowed is carefully logged (event-log, log4net, YMMV)
Typically, I'm all for the practice of letting uncaught exceptions simply "crash" the application by terminating the CLR. However, especially in server applications, this is sometimes not sensible. If one thread encounters a problem which is deemed non-fatal, there is no reason in ripping the whole process down, killing off all other running requests (WCF, for example, handles some cases this way).
You should usually still catch generic exceptions in a global handler (which is the perfect place to log unexpected Exceptions), but otherwise as said before, you should only catch specific exception types in other places if you plan to do something with them. The catch blocks should look for those exception types explicitly, not as the code you posted does.
I don't think this is such a bad thing in this case, but I also do something similar in my code with exceptions that can be safely ignored being caught and the rest being re-thrown. As noted by Michael's response, having each catch being a separate block could cause some readability issues which are prevented by going this route.
In regards to pointing this out to your colleague, I think you would have a hard time convincing them that this is the wrong way of doing things - even more so if they are stubborn - because of the potential readability issues with doing things the other way. Since this version is still throwing the generic exception's that can't be handled it is in keeping with the spirit of the practice. However, if the code was in line with the following:
try
{
// Do some work
}
catch (Exception ex)
{
if (ex is SoapException)
{
// SoapException specific recovery actions
}
else if (ex is HttpException)
{
// HttpException specific recovery actions
}
else if (ex is WebException)
{
// WebException specific recoery actions
}
else
{
throw;
}
}
Then I think you would have a bit more of a reason to be concerned as there is no point in doing work for a specific exception by checking for it in a general exception block.
the princeple is only to catch the exception you can handle.
such as, if you know how to deal with findnotfound, you catch the filenotfoundexception, otherwiese, do NOT catch it and let it be thrown to the upper layer.