We Keep Coding

c# and catching exceptions

Is it good practice to append a general exception catch when handling exceptions. an example may make this question clearer
try{
// do something
}catch(spomespecificException1 ex){
//logging and other stuff
}catch(spomespecificException2 ex){
//logging and other stuff
}catch(Exception ex){
//logging and other stuff
}
should i append the exception catch to the stack

You shouldn't be catching those exceptions at all, in general. Don't catch exceptions that you don't actually know how to handle.
"Handle" means fix. If you can't fix the problem, or if you can't add additional information, then don't catch the exception.

It depends on the situation and how far up the stack you want to let an exception bubble before being caught.
There are certainly reasons for and against but without details specific to your situation it's impossible to tell.
So, is there a general "rule" about it? no.

The code you have posted is fine if you want to do some specific handling relating to that exception - which you would want to do in one of the other catch blocks. A good example might be when you are trying to connect to an SQL database and you can handle the different error messages in different ways.
Also, remember there is a "finally" block you can add to the end to do all clean up (and common) handling code, but you will not get exception information in there

Exceptions do generally happen especially if your accessing something outside your application like connection errors etc.
If you want to filter out the type of exceptions and do something different for each particular type, then there's nothing wrong with doing this.
A better coding practice would be preventing these exceptions from happening in the first place.

What's the best way to handle the exceptions and how to deal with them in asp.net

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.

General Exception Handling Strategy for .NET

I’m used to having try/catch blocks in every method. The reason for this is so that I can catch every exception at the point of infraction and log it. I understand, from my reading and conversations with others, that this isn’t a popular view. One should only catch what one is prepared to handle. However, if I don’t catch at the point of infraction, then it would be possible to never log that infraction and know about it. Note: When I do catch and don’t handle, I still throw. This allows me to let the exception propagate to something that will handle it, yet still let me log it at the point of infraction.
So... How does one avoid try/catch in every method, yet still log the error at the point at which it occurred?

No, don't catch everything. Exceptions propagate higher up on the stack. All you have to do is make sure that the exception is caught before it gets to the top of the stack.
This means, for instance, that you should surround the code of an event handler with a try/catch block. An event handler may be the "top of the stack". Same for a ThreadStart handler or a callback from an asynchronous method.
You also want to catch exceptions on layer boundaries, though in that case, you might just want to wrap the exception in a layer-specific exception.
In the case of ASP.NET, you may decide to allow ASP.NET Health Monitoring to log the exception for you.
But you certainly don't ever need to catch exceptions in every method. That's a major anti-pattern. I would loudly object to you checking in code with that kind of exception handling.

You can see everything at stack trace - no need to try/catch every method.
Stick to few rules:
Use try/catch only if you want to use a custom exception type
Define a new exception type only if upper levels needs to know that
Try/catch at top level instead of doing that for each method

OK, having read all the answers saying you should have a single try/catch at the top level, I'm going to weigh in with an alternative view.
I wouldn't put a try/catch in every method, far from it. But I would use a try/catch around sections of code that I expected to fail (e.g. opening a file), and where I wanted to add additional information to the exception (to be logged higher up the chain).
A stack trace saying and a message saying "permission denied" might be enough to allow you as a programmer to figure out what went wrong, but my goal is to provide the user with meaningful information, such as "Could not open file 'C:\lockedfile.txt'. Permission denied.".
As in:
private void DoSomethingWithFile(string filename)
{
// Note: try/catch doesn't need to surround the whole method...
if (File.Exists(filename))
{
try
{
// do something involving the file
}
catch (Exception ex)
{
throw new ApplicationException(string.Format("Cannot do something with file '{0}'.", filename), ex);
}
}
}
I'd also like to mention that even the people saying "only have one try/catch" would presumably still use try/finally throughout their code, since that's the only way to guarantee proper cleanup, etc.

Catching and rethrowing an exception that you cannot handle is nothing more than a waste of processor time. If you can't do anything with or about the exception, ignore it and let the caller respond to it.
If you want to log every exception, a global exception handler will do just fine. In .NET, the stack trace is an object; its properties can be inspected like any other. You can write the properties of the stack trace (even in string form) to your log of choice.
If you want to ensure that every exception is caught, a global exception handler should do the trick. In point of fact, no application should be without one.
Your catch blocks should only catch exceptions that you know that you can gracefully recover from. That is, if you can do something about it, catch it. Otherwise, let the caller worry about it. If no callers can do anything about it, let the global exception handler catch it, and log it.

I definitely don't use a try catch wrapper around every method (oddly enough, I did when I first started but that was before I learned better ways).
1) To prevent the program from crashing and the users losing their info, I do this
runProgram:
try
{
container.ShowDialog();
}
catch (Exception ex)
{
ExceptionManager.Publish(ex);
if (MessageBox.Show("A fatal error has occurred. Please save work and restart program. Would you like to try to continue?", "Fatal Error", MessageBoxButtons.YesNo) == DialogResult.Yes)
goto runProgram;
container.Close();
}
container is where my application starts so this basically puts a wrapper around my entire app so that nothing causes a crash that can't be recovered. This is one of those rare instances where I don't really mind using a goto (it is a small amount of code and still quite readable)
2) I only catch exceptions in methods where I expect something could go wrong (such as a timeout).
3) Just as a point of readibility, if you have a try catch block with a bunch of code in the try section and a bunch in the catch section, it is better to extract that code to a well named method.
public void delete(Page page)
{
try
{
deletePageAndAllReferences(page)
}
catch (Exception e)
{
logError(e);
}
}

To do it at the point of occurrence, you would still need a try/catch. But you don't necessarily need to catch the exceptions everywhere. They propagate up the call stack, and when they are caught, you get a stack trace. So if a problem emerges, you can always add more try/catches as needed.
Consider checking out one of the many logging frameworks that are available.

I don't think you need to catch everything at the point of infraction. You can bubble up your exceptions, and then use the StackTrace to figure out where the point of infraction actually occurred.
Also, if you need to have a try catch block, the best approach I've heard is to isolate it in a method, as to not clutter the code with huge try catch blocks. Also, make as few statements as possible in the try statement.
Of course, just to reiterate, bubbling up your exceptions to the top and logging the stacktrace is a better approach than nesting try-catch-log-throw blocks all throughout your code.

I would look into using ELMAH for exception handling, which is pretty much a concept of "let exceptions happen". ELMAH will take care of logging them and you can even set it to email you when exceptions for a certain project reach or exceed a specific threshold. In my department, we stay as far away from try/catch blocks as possible. If something is wrong in the application, we want to know right away what the issue is so we can fix it, instead of repressing the exception and handling it in code.
If an exception happens, that means that something isn't right. The idea is to make your application do only what it should do. If it's doing something different and causing exceptions, your response should be to fix the reason it's happening, not let it happen and handle it in code. This is just my/our philosophy and it's not for everyone. But we've all been burned way too many times by an application "eating" an exception for some reason or another and no one knows that something is wrong.
And never, ever, EVER catch a general Exception. Always, always, always catch the most specific exception so that if an exception is thrown, but it's not the type you're expecting, again, you'll know because the app will crash. If you just catch (Exception e), then no matter what type of exception is thrown, your catch block will now be responsible for responding to every single type of exception that could possibly be thrown. And if it doesn't, then you run into the whole "eating" exceptions, where something is going wrong but you never know until it is likely too late.

How does one avoid try/catch in every method, yet still log the error at the point at which it occurred?
It depends on the hosting env. Asp.Net, WinForms, and WPF all have different ways of capturing unhandled exceptions. But once the global handler is passes an exception instance you can determine the point of throw from the exception, as each exception includes a stacktrace.

Realistically, avoid granular try/catches. Allow the exception to traverse upwards in the stack and be caught in as high a level as possible. If you have a specific point of concern, then place logging in the immediate catch, if you are worried about the exception cascading - although you would still be able to resolve these by drilling into the inner exceptions.
Exception handling should not be an afterthought. Make sure that you do it consistently. I have seen a lot of people put a broad try/catch from the beginning to the end of every method and catch the general exception. People think that this helps them get more information, when in fact, it doesn't. More is less in some cases, as less is more. I never get tired of the axiom that "Exceptions should be used to note exceptional behavior." Recover if you can, and try to reduce the number of overall exceptions. Nothing is more frustrating when you are trying to troubleshoot an issue and seeing hundreds of the same NullReferenceException, or similar, when something goes wrong.

Exceptions are implemented so that they have no cost unless thrown.
This means to me that performance ramifications is not a strong argument against. Exceptional conditions usually are ... exceptional.

What is the right way to pass on an exception? (C#) [duplicate]

This question already has answers here:
Best practices for catching and re-throwing .NET exceptions
(11 answers)
Closed 9 years ago.
I'm wondering what the correct way is to pass on an exception from one method to another.
I'm working on a project that is divided into Presentation (web), Business and Logic layers, and errors (e.g. SqlExceptions) need to be passed down the chain to notify the web layer when something goes wrong.
I've seen 3 basic approaches:
try
{
//error code
}
catch (Exception ex)
{
throw ex;
}
(simply rethrow)
try
{
//error code
}
catch (Exception ex)
{
throw new MyCustomException();
}
(throw a custom exception, so that a dependency on the data provider is not passed on)
and then simply
//error code
(not doing anything at all, letting the error bubble up by itself)
Naturally there's some logging happening in the catch block too.
I prefer number 3, while my colleague uses method 1, but neither of us can really motivate why.
What are the advantages/disadvantages of using each method? Is there a better method I don't know of? Is there an accepted Best Way?

If you do nothing you should simply let it go upper where some one will handle it.
You can always handle a part of it (like logging) and re-throw it. You can re-throw by simply sending throw; without having to explicit the ex name.
try
{
}
catch (Exception e)
{
throw;
}
The advantage to handle it is that you can ensure that some mechanism is there to notify you that you have an error where you do not suspect to have one.
But, in some case, let say a Third Party, you want to let the user handle it and when it's that case you should let it continue to bubble up.

I think you should start with a slightly different question
How do I expect other components to interact with exceptions thrown from my module?
If the consumers are quite capable of handling the exceptions thrown by the lower / data layers then quite simply do nothing. The upper layer is capable of handling the exceptions and you should only do the minimum amount necessary to maintain your state and then rethrow.
If the consumers cannot handle low level exceptions but instead need a bit higher level exceptions, then create a new exception class which they can handle. But make sure to pass on the original exception a the inner exception.
throw new MyCustomException(msg, ex);

The correct way to re-throw an exception in C# is like below:
try
{
....
}
catch (Exception e)
{
throw;
}
See this thread for specifics.

Only use try/catch blocks around exceptions you expect and can handle. If you catch something you cannot handle, it defeats the purpose of try/catch which is to handle expected errors.
Catching big exception is rarely a good idea. The first time you catch an OutOfMemoryException are you really going to be able to gracefully handle it? Most API's document the exceptions each method can throw, and those should be the only ones handled and only if you can gracefully handle it.
If you want to handle the error further up the chain, let it bubble up on its own without catching and rethrowing it. The only exception to this would be for logging purposes, but logging at every step is going to do an excessive amount of work. It is better to just document where exceptions your public methods can be expected to allow to bubble up and let the consumer of your API make the decision on how to handle it.

No one has yet pointed out the very first thing you should be thinking of: what are the threats?
When a back-end tier throws an exception, something terrible and unexpected has happened. The unexpected terrible situation might have happened because that tier is under attack by a hostile user. The last thing you want to do in that case is serve up to the attacker a detailed list of everything that went wrong and why. When something goes wrong on the business logic tier the right thing to do is to carefully log all information about the exception and replace the exception with a generic "We're sorry, something went wrong, administration has been alerted, please try again" page.
One of the things to keep track of is all the information you have about the user and what they were doing when the exception happened. That way if you detect that the same user always seems to be causing problems, you can evaluate whether they are likely to be probing you for weaknesses, or simply using an unusual corner of the application that wasn't sufficiently tested.
Get the security design right first, and only then worry about diagnosis and debugging.

I've seen (and held) various strong opinions about this. The answer is that I don't think there currently is an ideal approach in C#.
At one point I felt that (in a Java-minded way) the exception is part of the binary interface of a method, as much as the return type and parameter types. But in C#, it simply isn't. This is clear from the fact that there is no throws specification system.
In other words, you can if you wish take the attitude that only your exception types should fly out of your library's methods, so your clients don't depend on your library's internal details. But few libraries bother to do this.
The official C# team advice is to catch each specific type that might be thrown by a method, if you think you can handle them. Don't catch anything you can't really handle. This implies no encapsulation of internal exceptions at library boundaries.
But in turn, that means that you need perfect documentation of what might be thrown by a given method. Modern applications rely on mounds of third party libraries, rapidly evolving. It makes a mockery of having a static typing system if they are all trying to catch specific exception types that might not be correct in future combinations of library versions, with no compile-time checking.
So people do this:
try
{
}
catch (Exception x)
{
// log the message, the stack trace, whatever
}
The problem is that this catches all exception types, including those that fundamentally indicate a severe problem, such as a null reference exception. This means the program is in an unknown state. The moment that is detected, it ought to shut down before it does some damage to the user's persistent data (starts trashing files, database records, etc).
The hidden problem here is try/finally. It's a great language feature - indeed it's essential - but if a serious enough exception is flying up the stack, should it really be causing finally blocks to run? Do you really want the evidence to be destroyed when there's a bug in progress? And if the program is in an unknown state, anything important could be destroyed by those finally blocks.
So what you really want is (updated for C# 6!):
try
{
// attempt some operation
}
catch (Exception x) when (x.IsTolerable())
{
// log and skip this operation, keep running
}
In this example, you would write IsTolerable as an extension method on Exception which returns false if the innermost exception is NullReferenceException, IndexOutOfRangeException, InvalidCastException or any other exception types that you have decided must indicate a low-level error that must halt execution and require investigation. These are "intolerable" situations.
This might be termed "optimistic" exception handling: assuming that all exceptions are tolerable except for a set of known blacklisted types. The alternative (supported by C# 5 and earlier) is the "pessimistic" approach, where only known whitelisted exceptions are considered tolerable and anything else is unhandled.
Years ago the pessimistic approach was the official recommended stance. But these days the CLR itself catches all exceptions in Task.Run so it can move errors between threads. This causes finally blocks to execute. So the CRL is very optimistic by default.
You can also enlist with the AppDomain.UnhandledException event, save as much information as you can for support purposes (at least the stack trace) and then call Environment.FailFast to shut down your process before any finally blocks can execute (which might destroy valuable information needed to investigate the bug, or throw other exceptions that hide the original one).

I'm not sure that there really is an accepted best practices, but IMO
try // form 1: useful only for the logging, and only in debug builds.
{
//error code
}
catch (Exception ex)
{
throw;// ex;
}
Makes no real sense except for the logging aspect, so I would do this only in a debug build. Catching an rethrowing is costly, so you should have a reason for paying that cost other than just that you like looking at the code.
try // form 2: not useful at all
{
//error code
}
catch (Exception ex)
{
throw new MyCustomException();
}
This one makes no sense at all. It's discarding the real exception and replacing it with one that contains less information about the actual real problem. I can see possibly doing this if I want to augment the exception with some information about what was happening.
try // form 3: about as useful as form 1
{
//error code
}
catch (Exception ex)
{
throw new MyCustomException(ex, MyContextInformation);
}
But I would say in nearly all cases where you are not handling the exception the best form is to simply let a higher level handler deal with it.
// form 4: the best form unless you need to log the exceptions.
// error code. no try - let it percolate up to a handler that does something productive.

Normally you'd catch only exceptions which you expect, can handle and let application work in a normal way further. In case you'd like to have some additional error logging you'll catch an exception, do logging and rethrow it using "throw;" so the stack trace is not modified. Custom exceptions are usually created for the purpose of reporting your application specific errors.

always try-catch external resource calls?

Should I always wrap external resource calls in a try-catch? (ie. calls to a database or file system) Is there a best practice for error handling when calling external resources?

Catch only exceptions that you can handle. So for example when using external resources, the best practice is to catch specific exceptions that you know you can handle. In case of files this can be (IOException, SecurityException, etc), in case of Database the exception can be SqlException or others.
In any case, don't catch exceptions that you don't handle, let them flow to a upper layer that can. Or if for some reason you do catch exceptions but don't handle them, rethrow them using just throw; (which will create an rethrow IL op, as opposed to trow).
In case of using resources that you don't know what type of exceptions might throw, you are kind of forced to catch the general exception type. And in this case the safes thing would be to use the said resources from a different app domain (if possible), or let the exception bubble up to top level (ex UI) where they can be displayed or logged.

I think there are three reasons to have a catch block:
You can handle the exception and recover (from "low level" code)
You want to rewrap the exception (again, from "low level" code)
You're at the top of the stack, and while you can't recover the operation itself, you don't want the whole app to go down
If you stick to these, you should have very few catch blocks compared with try/finally blocks - and those try/finally blocks are almost always just calling Dispose, and therefore best written as using statements.
Bottom line: It's very important to have a finally block to free resources, but catch blocks should usually be rarer.

> Know when to set up a try/catch block. For example, you can programmatically check for a condition that is likely to occur without using exception handling. In other situations, using exception handling to catch an error condition is appropriate.
That's what I found and it makes sense to me.. Check manually for the obvious things, let try-catch do the rest..

Eric Lippert has a good blog on this, here.
There is no point (except for "vexing" (see blog)) catching an exception unless you can do something useful; and in most cases you simply can't - so let it bubble (your UI should obviously cleanse and display something).
You might, however, have a "try/finally" to deal with resource management. Or even cleaner, a "using" block to do the same.

I think the absolute answer is completely conditional (how control do you have over the environment, what is the expected balance between performance and consistency and many others I'm sure), but generally speaking I always do, choosing the safety over the potentially slower performance.

it always depend on what you want to achieve. A server not responding might be serious enough to stop all what the routine what doing, and the exception should be thrown to the caller.
In other cases, you don't care whether you failed to update the db or not. Then consuming the exception is OK.
Obviously, you don't want to show the stack trace to your end user, though, so you'll need to catch it somewhere.