Related
If my try throws, and my finally throws, how can I hold onto both exceptions, in a single chain, so that I can let them propagate up the stack naturally instead of requiring an intermediate logging facility?
I thought this might help:
Exception exCause = null;
try {
// do something risky
} catch ( Exception ex ) {
exCause = ex;
throw;
} finally {
//check for an important error condition
// if error: throw new ApplicationException("finally failed", exCause);
}
But I wasn't sure if it might break something in the framework.
Obviously it still has the problem of two pre-existing exceptions that can't be merged if I call something that throws an exception in the finally, but I thought it might help a little bit to alleviate the need for every library in a system to have access to the same tracing routines.
Maybe you could have a look at the System.AggregateException class
AggregateException is used to consolidate multiple failures into a single, throwable exception object.
http://msdn.microsoft.com/en-us/library/system.aggregateexception.aspx
That might help you
This situation is best avoided by writing your finally handler in such a way that it won't throw an exception.
Neither C++, nor Java, nor .Net, has historically offered any good way of dealing with exceptions that occur in stack-unwinding cleanup code. This is unfortunate, because in many cases exceptions which occur in cleanup code are more important to the behavior of the program than exceptions which occur in main-line code, but are less helpful for diagnostic purposes (especially since a clean-up exception may occur as a result of a main-line exception).
It is possible in VB.net to write a try/finally block in such a way that the finally code will know what exception, if any, caused code to leave the try block, without claiming to catch or handle the exception in question. Unfortunately, that isn't possible in C# though it's possible for code in C# to achieve such an effect by calling a wrapper written in vb.net; the use of such a wrapper may be helpful even in vb.net since the code necessary to achieve the proper semantics is a bit icky. The using block in both languages could be improved if it could use an interface something like IDisposableDuringException { void Dispose(Exception ex);} which would be called if the Dispose was being executed during stack unwinding for an exception. That would allow code in the Dispose method to include the supplied exception in any exception that it might throw, and would also for the possibility of having dispose behavior vary based upon whether an exception was thrown (e.g. transaction scopes should clearly perform a rollback if they exit via exception--that's the point of them after all--but requiring them to implicitly perform a rollback when exited via normal means is icky).
With regard to what should happen if an exception is thrown during finally cleanup, I would suggest that whether or not an exception is pending, the proper behavior should be to throw something like a custom CleanupFailureException. Even if the calling code would have been prepared to catch e.g. InvalidOperationException thrown from the main-line, a successful completion of the main-line followed by an InvalidOperationException during cleanup will likely represent a situation different from the one the code was prepared to handle, and should thus use a different exception type.
Incidentally, there's often some debate about whether code should assume unexpected exceptions are fatal or non-fatal. I would posit that in many cases the proper approach would be to have stack unwinding from unexpected exceptions expressly invalidate the objects that were being worked with, such that all future operations with those objects will cause exceptions. This would have the effect that if an unexpected exception may corrupt a critically-important object the program may die off before doing any damage, but simultaneously allow for the possibility that if an exception corrupts an object which isn't going to be needed anyway (e.g. if an unexpected problem occurs while loading a document from a damaged file, and in the attempt to load the document the data structures associated with it got hopelessly corrupted, then if the code can abandon the partially-loaded document proceed sensibly without it, such corruption need not affect anything else.
My understanding is based on this long, but fantastic, article which supports the behavior listed in the C# specification.
The CLI standard (EMCA-335) shows that if there is no suitable catch, the runtime should terminate immediately. The .NET runtime does not do this, instead it seems to lean toward the behavior of the C# specification (EMCA-334).
First, I find it strange that a language specification is appears to be defining framework behavior. Secondly, They seem to contradict.
Do they contradict each other, or am I getting the wrong meaning of the document?
Does a runtime have to go about exception handling in this way to be compliant with the standard?
As an optional question, which one is the "correct" one, as in, if I were to write my own implementation of the CLI which one should I use? Note that EMCA-335 (CLI) document was updated two months ago, where EMCA-334 (C#) was updated back in 2006.
ECMA-335 Partition I Section 12.4.2.5
When an exception occurs, the CLI searches the array for the first protected block that
Protects a region including the current instruction pointer and
Is a catch handler block and
Whose filter wishes to handle the exception
If a match is not found in the current method, the calling method is searched, and so on. If no match is found the CLI will dump a stack trace and abort the program.
If a match is found, the CLI walks the stack back to the point just located, but this time calling the finally and fault handlers. It then starts the corresponding exception handler.
C# Specification §15.9.5 and §15.10 (§8.9.5 and §8.10 on MSDN)
The main difference between it and the CLI standard, is that whether or not a catch block is found, the application will not just exist, but will still unwind the stack, and take care of finally handlers.
I would suggest reading the standard itself to get a better meaning of this, since below is a very crude summary. It outlines step-by-step how a try statement is executed with each possible scenario.
In the function that raises the exception:
Looks for a matching catch clause in each try statement
Executes the catch statement if it exists
A finally block is executed if it exists
If there was no handler, the above steps are repeated in the calling function
If the exception processing terminates all function member invocations in the current thread, indicating that the thread has no handler for the exception, then the thread is itself terminated. The impact of such termination is implementation-defined.
There's no conflict here. The C# language specification is worded like this:
If the try statement has no catch clauses or if no catch clause matches the exception:
• If the try statement has a finally block, the finally block is executed.
• The exception is propagated to the next enclosing try statement.
Bullet 2 here specially doesn't say what happens when there is no next enclosing try statement. For that, turn to the end of 8.9.5:
If the exception processing terminates all function member invocations in the current thread, indicating that the thread has no handler for the exception, then the thread is itself terminated. The impact of such termination is implementation-defined.
It certainly is implementation-defined. Beyond the Ecma 335 spec, the exception handling policy is a configurable item in the Microsoft CLR. Controlled by ICLRPolicyManager::SetActionOnFailure(). In turn configurable in the default host with the <legacyUnhandledExceptionPolicy> app.exe.config file element. The default for CLR version 2.0 and up is to immediately terminate the program.
This is otherwise fairly unproductive biblical hermeneutics. None of this should come as a surprise to a C# programmer, especially given how easy it is to test.
I think this might just be a matter of a vague wording.
If a match is not found in the current method, the calling method is searched, and so on. If no match is found the CLI will dump a stack trace and abort the program.
Okay, that's true in C#. We all know that if we don't have a catch then an exception will bring down our program.
If a match is found, the CLI walks the stack back to the point just located, but this time calling the finally and fault handlers. It then starts the corresponding exception handler.
And that matches what we know from C# too. If there are some finally (we don't get to see fault) blocks to deal with as we go up the stack from the exception being thrown until our catch block, they get processed, but it stops there and goes no further up the stack.
A lot hangs on how we read the "If" that starts that second excerpt I just quoted. You're reading it as "if ... then ... otherwise no such thing". It could be read though as the first excerpt identifying the point in the stack that will be walked to: If there was a catch, then it's walked to that point. If there is no catch, then it's walked to the very top of the stack and we get a dump and abort. The finally handlers (and fault handlers) still get called, but the point is not that of a matching catch handler.
Your reading is the most literal, and mine the one that stretches things a bit. However, mine does match with the description of finally elsewhere in the same standard, most closely
The cited article in the O.P. has an incorrect underlying assumption:
Of course, we can’t talk about managed exceptions without first considering Windows
Structured Exception Handling (SEH). And we also need to look at the C++ exception
model. That’s because both managed exceptions and C++ exceptions are implemented
on top of the underlying SEH mechanism, and because managed exceptions must
interoperate with both SEH and C++ exceptions.
The CLR standard (ISO 23271/ECMA 335) is intentionally platform-agnostic. Microsoft's implementation is one of many possible implementations (Mono, of course, being another).
Interoperability with Windows Structured Exception Handling and C++ exception handling is, I'm pretty sure, Microsoft's choice and not an ISO 23271 requirement.
I don't need a lesson in switching from recursive to non-recursive means, I just want to know why we can't deal with this type of exception. Regardless, I'm using recursive functions on very large lists.
I have written code to attempt to catch StackOverFlowExceptions:
try { recursiveFxn(100000); }
catch(Exception){}
private void recursiveFxn(int countdown)
{
if (countdown > 0)
recursiveFxn(countdown - 1);
else
throw new Exception("lol. Forced exception.");
}
But still I get program crashes (in both NUnit and a webpage I'm running). Why isn't the exception caught?
Since .NET Framework 2.0, StackOverflowException cannot be caught. This is because it is considered a bad practice. Quoting the MSDN documentation:
Starting with the .NET Framework
version 2.0, a StackOverflowException
object cannot be caught by a try-catch
block and the corresponding process is
terminated by default. Consequently,
users are advised to write their code
to detect and prevent a stack
overflow. For example, if your
application depends on recursion, use
a counter or a state condition to
terminate the recursive loop.
Now, the only way to catch a StackOverflowException is when it was thrown by user code, as explained in a blog by Jared Parsons. Other than that, by hosting the CLR, you can handle (but not catch) a StackOverflowException and devise a way to let the execution of your program continue.
Note that because the stack is unwound when an exception occurs, in pre-2.0 versions of .Net the stack would actually be much shorter when the StackOverflowException is handled, making it possible to do so without generating another StackOverflowException.
You can't catch a stack overflow exception because when it happens it kills the thread dead. Try... catch... is performed by the same thread so that won't work. There may be some lower level APIs that you could P/Invoke and have another thread catch it.
There may also be some lower level APIs to change the maximum stack size, but I don't see anything in the .NET Framework to help with that so again you would need to P/Invoke something.
I've dealt with instances where I would throw/rethrow an exception knowing that the code surrounding it would catch the specific exception. But is there any time you would want to throw an exception, knowing that it wouldn't be caught?
Or at least, NOT catch an exception?
Exceptions immediately halt the application unless their handled right? So I guess I'm asking if you would ever want to purposely let your application die?
If your application is primarily going to be used by other clients and is not standalone, it generally makes sense to throw exceptions if a condition arises that you don't know how to (or don't want to) handle, and there's no sensible way for you to recover from it. Clients should be able to decide how they want to handle any exceptions that you might throw.
On the other hand, if your application is the endpoint, throwing an exception essentially becomes a notification mechanism to alert people that something has gone terribly wrong. In such cases, you need to consider a few things:
How important is the continued running of the application? Is this error really unrecoverable? Throwing an exception and terminating your program is not something you want to be doing on the space shuttle.
Are you using exceptions as a proxy for real logging? There's almost never a reason to do this; consider a real logging mechanism instead. Catch the exception and have the logger work out what happened.
What are you trying to convey by throwing the exception yourself? Ask yourself what the value in throwing a new exception is, and consider carefully whether there isn't a better way to do what you want.
Not catching an exception may leave resources in a bad state. If you don't gracefully exit, things are generally not cleaned up for you. Make sure you understand what you're doing if you need to do this -- and if you're not going to catch it, at least consider a try-finally block so you can do some tidying up.
There's a very good rule that I came across a while ago:
Throw an exception when a method can't do what its name says it does.
The idea is that an exception indicates that something has gone wrong. When you are implementing a method, it is not your responsibility to be aware of whether it will be used correctly or not. Whether the code using your method catches the exception or not is not your responsibility, but the responsibility of the person using your method.
Another rule to follow is:
Don't catch an exception unless you know what you want to do with it.
Obviously, you should include cleanup code in a try...finally block, but you should never just catch an exception just for the sake of catching it. And you should never swallow exceptions silently. While there are occasions when you may want to catch all exceptions (e.g. by doing catch (Exception ex) in C#), these are fairly uncommon and generally have a very specific technical reason. For example, when you are using threads in .NET 2.0 or later, if an exception escapes from your thread, it will cause the entire application domain to unload. In these cases, however, at the very minimum you should log the exception details as an error and provide an explanation in the comments.
Sure. For example, if you're trying to load some bytes into a string in Java:
try {
String myString = new String(byteArray, "UTF-8");
} catch (UnsupportedEncodingException e) {
// Platform doesn't support UTF-8? What is this, 1991?
throw new RuntimeExceptione(e);
}
In this case, there is no graceful degradation, the platform simply can't support the operation desired. You can check for this condition at initialization all you want, but the constructor for String still throws this exception, and you have to deal with it. Either that, or use Charset.forName() :)
Generally, and certainly in early iterations of your application, don't catch the exception. More often than not, the recovery from an exception will require a business rule of some sort, and, more often than not, those business rules are not defined for you. If you "handle" the exception instead of letting the application die then you will most likely be inventing business rules for your customer. Not good.
The general pattern of catching every exception just for the sake of catching it has caused me more headaches than I can count. It usually happens that someone puts some sort of generic exception handling code throughout the application, which inevitably ends up hiding a bug or creating some behavior that is unwanted. (incidentally, catching and then not rethrowing is even worse.)
So, I'd suggest that you ask instead: "When should I catch an exception?"
Here's the thing ... it is about "layers", or "encapsulation", or "low coupling". At some place in your codebase, you're writing a method to do something. Say it's a public method. Therefore, it should not assume much or anything about the caller ... rather, it should merely do the job it is supposed to do, regardless of who is calling it and what context the caller is in.
And if, for some reason, it cannot complete its job, then it needs to tell the caller "Sorry, I couldn't do that, and here's why". Exceptions are an excellent mechanism to let it tell the caller that (not the only mechanism, but the best mechanism I've ever seen for most cases).
So, when you throw the exception, you have no idea whether it will be caught or not ... because you're exposing a public method and you have no idea who might choose to call it and why.
The catching of the exception is the job of the "context". For example, say you're writing a library with public methods that might throw exceptions. Then, say you're using that library from a Windows Forms app. The Windows Forms app might catch exceptions and show a message box to the user.
But later, you might use the same library from a Windows Service. The Service would be more likely to catch the exception, log it, return an error to the original caller, but keep running so it can process further requests.
So the exception is like a contractual agreement between the caller and the provider. The provider says, "I'll either do the job or tell you why I can't. What you do from there is your own business." And the caller says, "OK, if you can't do the job, just tell me why, and I'll decide what to do in that case."
But is there any time you would want to throw an exception, knowing that it wouldn't be caught?
I would say that if you're manually throwing an exception, most of the time you don't know if it will be caught. If you knew it would be caught you could just handle it yourself rather than throwing the exception in the first place.
To be fair, I suppose that depends in part on the kind of programming you're doing, and sometimes the same programmer ends up building both the library and the code that consumes said library.
Would you ever NOT catch an exception?
If you didn't expect/weren't aware an exception could be thrown. But putting that aside and assuming you are aware of the exception, sometimes you know about it at one tier but know the next tier up is the more appropriate place to handle it.
It depends on the type of application. Web applications can continue running even after exceptions have bubbled up to the execution context.
It is common practice to 'throw/rethrow' an exception if you catch the exception at a level where it can't be dealt with. But, you would almost always add context to the issue, at the very least add some logging at the higher level to say that it was caught and rethrown.
for example
A calls B calls C (throws exception)
B catches/rethrows
A catches.
In this case, you would want B to add some logging so that you can differentiate between B generating and throwing an error, and C generating and throwing an error. That would allow you a greater ability to debug and fix problems later.
In general you will almost NEVER want an exception to kill your program. The best practice is to catch the except and exit gracefully. This allows you to save any currently open information and release resources that are being used so they don't become corrupted. If you intend to exit, you can create your own 'core-dump' information report that includes the things you were doing when you caught the fatal exception.
If you let the exception kill your process you are eliminating your chance to get custom tailored crash information, and you are also skipping the part where you provide the user with a friendly error message and then exit.
So, I would recommend ALWAYS catching exceptions, and never voluntarily letting them run amok in your program.
EDIT
If you are writing a library, you have to choose ahead of time whether your function will throw an exception, or be exception safe. In those cases, sometimes you will throw an exception and have no idea if the calling party will catch it. But in that case, catching it is not your responsibility, as long as the api declares that the function could throw exceptions.
(I'm looking for a word that means 'could possibly throw exception'... anyone know what it is? It's going to bug me all day.)
Firstly, there absolutely are situations where it is better to not catch an exception.
Sometimes, an exception can sometimes tell you that your program is in an unknown state. There are a number of exceptions where this is pretty much intrinsically true given the exception type. A NullReferenceException essentially tells you "there is a bug". And by catching such an exception, you may hide the bug, which sounds good in the short term, but in the long term you'd be happier to fix it. The product may not crash, but it certainly won't have the expected behaviour.
But this is also true for exception types we invent for ourselves. Sometimes, the fact that exception A has been thrown should be "impossible" - and yet it has happened, so there's a bug.
Also, something very important happens when you catch an exception: the finally blocks for the whole call stack inside the try block (and anything it calls) will be executed. What do those finally blocks do? Well, anything. And if the program is in an unknown state, I really do mean anything. They could erase valuable customer data from the disk. They could throw more exceptions. They could corrupt data in memory, making the bug impossible to diagnose.
So when an exception indicates an unknown state, you don't want to run any more code, so whatever you do, don't catch the exception. Let it fly past, and your program will terminate harmlessly, and Windows Error Reporting will be able to capture the state of the program as it was when the problem was originally detected. If you catch the exception, you will cause more code to execute, which will screw up the state of the program further.
Secondly, should you throw an exception knowing it won't be caught? I think that question misunderstands the nature of reusable methods. The whole idea of a method is that it has a "contract" that it follows: it accepts certain parameters and returns a certain value, plus also it throws certain exceptions under certain conditions. That's the contract - it's up to the caller what they do with it. For some callers, exception A might indicate a recoverable condition. For other callers, it might indicate a bug. And from what I said above, it should be clear that if an exception indicates a bug, it must not be caught.
And if you're wondering what this means for the Microsoft Enterprise Library's Exception Handling Block: yes, it's pretty broken. They tell you to catch (Exception x) and then decide whether to rethrow based on your policy; too late - the finally blocks have already executed by that point. Don't do that.
You probably wouldn't want an uncaught exception anywhere where the end-users can see it, but it is often acceptable to let clients of your API (other programmers) decide how to handle exceptions.
For example, suppose you are designing a Java class library. You expose a public method that takes in a String. In your application, a null input value would cause an error. Instead of handling the error yourself, it would be acceptable to check for a null value, then throw an IllegalArgumentException.
You must, of course, document that your method throws this exception in this circumstance. This behavior becomes part of your method's contract.
It depends on what you mean by 'being caught'. Something, somewhere eventually catches the exception whether it be the underlying OS or something else.
We have a workflow system that executes job plans comprised of individual jobs. Each job runs a unit of code. For some of the exceptions, we don't want to handle them in the code but throw it up the stack so that the external workflow system catches it (which happens completely outside of the thrower's process).
If you're writing the entire application, then your reasons are your own. I can think of a few situations where you might want to throw the exception and let the app die, most of them are not very good reasons though.
The best reason is usually when debugging. I frequently disable exceptions while debugging to allow me to know better where something is failing. You can also just turn on thrown exception breaks in the debugger if you're running it on a machine with the debugger.
Another possible reason is when continuing after an exception is thrown doesn't make sense or would result in possible irrecoverable data corruption or worse (think Robots with laser beams, but then you should be damn sure your applicaiton deals with these situations IMO, crashing the program is just the lazy way).
If you're writing API code, or Framework code that you won't use yourself, then you have no idea if someone will catch your exceptions.
Yup, it's my ONLY opportunity to slap the developer consuming the service/object to tell them "Ur dO1n it WrOnG!!!!".
That and getting rid of possibilities that you don't want to permit or are seemingly "impossible". Apps that catch all exceptions and continue are just a walled garden surrounded by chaos.
If I need a moderately large system that is somehow processing data in what I believe to be a consistent manner.
And
Somewhere along the line, I detect that the application's state has become inconsistent.
And
The system doesn't (yet) know how to fix the inconsistency and recover gracefully
Then, yes, I would throw an exception with as much detail as possible and cause the application to die as quickly as possible, to avoid doing any further harm to the data. If it can be recovered, it'd be important not to exacerbate the problem by trying feebly to cover up the mess.
Later along the line, once the chain of events that led to the inconsistency is better understood, I higher facility can catch that exception, repair the state, and continue with minimal interruption.
A library will often throw exceptions based on defensive programming checks, should a condition arise that shouldn't have been allowed to arise by the application code. Applications code will often be written such that most of those invalid conditions will never arise, and therefore the exceptions will never be thrown, so there's no point catching them.
Depending on language (I'm mostly thinking in terms of C++ rather than C#, and not that clear what the differences are) the effect of an uncaught exception actually being thrown is probably the same as what used to be done in the days before exceptions were invented. A common policy for defensive programming in C libraries, for example, was to terminate the program immediately with an error message.
The difference is that if the exception throw does turn out to be possible (hopefully this will be discovered through unit testing), it is often relatively easy to add an exception handler that can recover from the problem in a more constructive way. You don't have to rewrite the library, or add complex checks in application code to ensure the condition cannot arise before the exception-throwing call is made.
I have quite a few exception throws that are never caught. They are all for defensive purposes, and while being uncaught is bad for an exception that does happen, this only ever happens during development and testing, for error conditions I failed to consider in the application code so far. And when it happens, it is unusual for the fix to be awkward - no need for a large-scale refactoring, no need for the applications code to be massively complicated with error condition checks, just a catch clause with a relatively simple recovery or "I'm sorry, Dave, I'm afraid I can't do that." without failing out the whole app.
We're reviewing one of the company's system's exception handling and found a couple of interesting things.
Most of the code blocks (if not all of them) are inside a try/catch block, and inside the catch block a new BaseApplicationException is being thrown - which seems to be coming from the Enterprise Libraries.
I'm in a bit of a trouble here as I don't see the benefits off doing this. (throwing another exception anytime one occurs)
One of the developers who's been using the system for a while said it's because that class's in charge of publishing the exception (sending emails and stuff like that) but he wasn't too sure about it.
After spending some time going through the code I'm quite confident to say, that's all it does is collecting information about the environment and than publishing it.
My question is:
- Is it reasonable to wrap all the code inside try { } catch { } blocks and than throw a new exception? And if it is, why? What's the benefit?
My personal opinion is that it would be much easier to use an HttpModule, sign up for the Error event of the Application event, and do what's necessary inside the module. If we'd go down this road, would we miss something? Any drawbacks?
Your opinion's much appreciated.
Never1 catch (Exception ex). Period2. There is no way you can handle all the different kinds of errors that you may catch.
Never3 catch an Exception-derived type if you can't handle it or provide additional information (to be used by subsequent exception handlers). Displaying an error message is not the same as handling the error.
A couple of reasons for this, from the top of my head:
Catching and rethrowing is expensive
You'll end up losing the stack trace
You'll have a low signal-to-noice ratio in your code
If you know how to handle a specific exception (and reset the application to pre-error state), catch it. (That's why it's called exception handling.)
To handle exceptions that are not caught, listen for the appropriate events. When doing WinForms, you'll need to listen for System.AppDomain.CurrentDomain.UnhandledException, and - if your doing Threading - System.Windows.Forms.Application.ThreadException. For web apps, there are similar mechanisms (System.Web.HttpApplication.Error).
As for wrapping framework exceptions in your application (non-)specific exceptions (i.e. throw new MyBaseException(ex);): Utterly pointless, and a bad smell.4
Edit
1 Never is a very harsh word, especially when it comes to engineering, as #Chris pointed out in the comments. I'll admit to being high on principles when I first wrote this answer.
2,3 See 1.
4 If you don't bring anything new to the table, I still stand by this. If you have caught Exception ex as part of a method that you know could fail in any number of ways, I believe that the current method should reflect that in it's signature. And as you know, exceptions is not part of the method signature.
If I am reading the question correctly, I would say that implementing a try / catch which intercept exceptions (you don't mention - is it catching all exceptions, or just a specific one?) and throws a different exception is generally a bad thing.
Disadvantages:
At the very least you will lose stack trace information - the stack you will see will only extend to the method in which the new exception is thrown - you potentially lose some good debug info here.
If you are catching Exception, you are running the risk of masking critical exceptions, like OutOfMemory or StackOverflow with a less critical exception, and thus leaving the process running, where perhaps it should have been torn down.
Possible Advantages:
In some very specific cases you could take an exception which doesn't have much debug value (like some exceptions coming back from a database) and wrap with an exception which adds more context, e.g id of the object you were dealing with.
However, in almost all cases this is a bad smell and should be used with caution.
Generally you should only catch an exception when there is something realistic that you can do in that location- ie recovering, rolling back, going to plan B etc. If there is nothing you can do about it, just allow it to pass up the chain. You should only catch and throw a new exception if there is specific and useful data available in that location which can augment the original exception and hence aid debugging.
I'm from the school of thought where try/ catch blocks should be used and exceptions not rethrown. If you have executing code which is likely to error then it should be handled, logged and something returned. Rethrowing the exception only serves the purpose to re-log later in the application life cycle.
Here's an interesting post on how to use a HttpModule to handle exceptions: http://blogs.msdn.com/rahulso/archive/2008/07/13/how-to-use-httpmodules-to-troubleshoot-your-asp-net-application.aspx and http://blogs.msdn.com/rahulso/archive/2008/07/18/asp-net-how-to-write-error-messages-into-a-text-file-using-a-simple-httpmodule.aspx
Check out ELMAH. It does what you're talking about. Very well.
When I create libraries I try to always provide a reduced number of exceptions for callers to handle. For example, think of a Repository component that connects to a sql database. There are TONS of exceptions, from sql client exceptions to invalid cast exceptions, that can theoretically be thrown. Many of these are clearly documented and can be accounted for at compile time. So, I catch as many of them as I can, place them in a single exception type, say a RepositoryException, and let that exception roll up the call stack.
The original exception is retained, so the original exception can be diagnosed. But my callers only need to worry about handling a single exception type rather than litter their code with tons of different catch blocks.
There are, of course, some issues with this. Most notably, if the caller can handle some of these exceptions, they have to root around in the RepositoryException and then switch on the type of the inner exception to handle it. Its less clean than having a single catch block for a single exception type. I don't think thats much of an issue, however.
Sounds like the exception that is thrown should not have been implemented as an exception.
Anyway, I would say that since this BaseApplicationException is a general all-purpose exception, it would be good to throw exceptions that are more context-specific. So when you are trying to retrieve an entity from a database, you might want an EntityNotFoundException. This way when you are debugging you do not have to search through inner exceptions and stack traces to find the real issue. If this BAseApplicationException is collecting information on the exception (like keeping track of the inner exception) then this should not be a problem.
I would use the HttpModule only when I could not get any closer to where the exceptions are actually happening in code. You do not really want an HttModule OnError event that is a giant switch statement depending on BaseApplicationexception's error information.
To conclude, it is worth it to throw different exceptions when you can give more specific exceptions that tell you the root of the problem right off the bat.
From my experience, catch the exception, add the error to the Server (?) object. This will allow .NET to do what ever it needs to do, then display your exception.