I'm trying to wrap my head around this. According to this page on Using statements:
The using statement ensures that Dispose is called even if an exception occurs while you are calling methods on the object. You can achieve the same result by putting the object inside a try block and then calling Dispose in a finally block; in fact, this is how the using statement is translated by the compiler.
But on this page about Try-Finally blocks it states:
Within a handled exception, the associated finally block is guaranteed to be run. However, if the exception is unhandled, execution of the finally block is dependent on how the exception unwind operation is triggered.
So how can a Using statement be guaranteed to call the Dispose method in the event of an exception if it translates to a Try-Finally that is not guaranteed to call the finally statement?
It really does behave like a try/finally - so if the application terminates, the resource may not be disposed... and that's usually okay, because normally disposal is for releasing resources held by the process... and the OS will tidy those up anyway on process death. (That's not to say the Dispose method won't be called... it's just the same as with a normal try/finally.)
Obviously if you've got a "lock file" on the file system or something like that, that would be a problem - but you'd have the same problem in the face of a power cut etc.
There are exceptions from which a program can't recover, in that case finally block will not execute. For example Stack overflow exception or Out of memory exception.
If the exception unwind only ends with an unhandled exception that crashes the program then finally blocks do not run as execution ceases.
tl;dr
Finally blocks are only run on success or on handled exception.
Related
I created .NET Core console app and added some async magic there. But somehow all exception popups from async methods appear in top-level main method (see screenshot). It makes debugging hard. How i can move them to appropriate places?
How it's now:
I want to move it here:
The debugger will only break when the Exception is unhandled, meaning that there is no further catch block around it. This is also where it will show up in production logs!
Async methods have an "implicit" try-catch around the function. When an exception is thrown in an async method, it is caught and only reintroduced ("re-thrown") into the call stack when the method is awaited (either via the await keyword or through .Result / .GetAwaiter().GetResult()).
This creates a "nice" async call stack that includes the complete flow of the exception over multiple async methods.
As has been mentioned in the comments, you can set the VS debugger to break when an exception is thrown, but this will also break everywhere an exception is about to be handled correctly (it will break before any catch block can run).
I'm working on improving the exception-safety of a piece of code and I realized that a raised ThreadAbortException may cause undesired resource leaks, even when guarding resources with the C# using construct. For instance, consider the following code (which could be running in a separate thread).
using (TextWriter writer = CreateWriter(filename))
{
// do something with the writer.
}
TextWriter CreateWriter(string filename)
{
return new CustomWriter(File.OpenWrite(filename));
}
If the thread running this code is abnormally terminated, then I would like the file handle referenced by filename to be closed immediately. Can I do this without replacing the use of the using construct with a try/finally block?
My assumption is that ThreadAbortException may be raised at anytime, which means I should pay attention to what is happening between statements. While I can guard against the exception in CreateWriter with a try/finally block, the using construct won't do the same until after the expression in the parenthesis is evaluated, meaning the file resource is left open if the exception occurs immediately after CreateWriter returns.
I understand that a finalizer will ultimately release the file handle, but I am wondering if there is a deterministic way to address this issue without catching ThreadAbortException in each place that CreateWriter is used.
Yes, the deterministic way of preventing this is by not using Thread.Abort. Ever. Signal to your threads that is is time to stop, and let them terminate gracefully. Thread.Abort is a great big red-herring, placed in the API solely to trip you up. ;)
http://www.interact-sw.co.uk/iangblog/2004/11/12/cancellation
There is a tradeoff.
Be sure to close all resources immediately, even in the presence of ThreadAbortException
Have simpler code, but temporarily leak resources if Abort() is called
I assume that you are not calling Abort, and just want a way to be safe if someone else does. If you are calling Abort, then I'd advise that you don't. This isn't the only problem you will run into. There are other problems with Abort in the documentation.
#2 is a valid choice because callers of Abort() should expect this.
If you want to choose #1, then I don't think even a simple try/catch will help. If the ThreadAbortException can happen everywhere, then it can still happen after the file is opened (inside File.OpenWrite()) and before you can assign it to a variable that you can call Dispose() on -- you will have the same problem as using in your code.
You need semantics like
using (var handle = GetUnOpenedHandle()) {
handle.Open(); // this can't involve assignment to any field of handle
}
I'm not sure this is possible.
In many cases (but definitely not all) you could guard against a ThreadAbortException. Most of the critical code in the .NET BCL does this fairly well already. The problem is that it is really hard to get right. And for this reason most people recommend, and rightly so, to avoid aborting threads. Starting in version 2.0 the CLR made thread aborts a lot more tolerable and introduced a new set of APIs to help code authors guard against them. Take a look at Constrained Execution Regions for an in depth look at how all of this works.
I believe you are correct about your concerns with the example of the using block. For constrained execution regions to work correctly the out-of-band (asynchronous) exception must occur from within a try block. But, because of the way using expands out the expression is evaluated outside of the try block. Contrast that with the expansion of the lock block which evaluates the expression from within the try block. Well, that is true with version 4.0 of the framework anyway and that was changed specifically to guard against these exceptions.
So the question is why was the same change not made with the using block. According to Joe Duffy this was an acceptable omission because the assumption is that thread aborts should always be followed by a termination of the AppDomain which would fire off the finalizers anyway.
So yes. Your code is not tolerant of out-of-band (asynchronous) exceptions. But, the prevailing wisdom from those smarter than me is that it should not have to be.
A thread abort is most often used in the case of a fatal error, so your response should probably be to let your application terminate. If your trying to stop your own threads cleanly, use Thread.Join().
I have background threads in my application. When one of the threads gets an exception that is not handled the entire CLR exits.
Is it normal behavior, or is it a bug in the CLR?
I would expect that the thread will exit but the CLR will continue working.
The default behavior in .NET applications is to exit whenever an unhandled exception occurs. When an exception goes unhandled, the program is in an unknown and possibly unsteady state. Just because it happened in a background thread doesn't mean that the error won't affect the rest of the program. The most prudent course for the runtime in that situation is to dump the program.
You might look into AppDomain.CurrentDomain.UnhandledException, which will allow you to catch unhandled exceptions and react accordingly. A better solution is to wrap your thread proc with a try...catch. But only have it handle those exceptions it knows how to handle. Doing this:
void MyThreadProc()
{
try
{
// ...
}
catch
{
// handle all exceptions
// This is a BAD idea
}
}
Is a really bad idea, because it can mask exceptions that you really do want to be propagated to the main program.
Your expected behavior used to be the behavior back in 1.1. It was generally considered to have been a bad idea. When you have an unhandled exception in any thread, your process can be left in an inconsistent state. Updates to shared data may be partially applied, etc. The runtime doesn't have the information to safely handle this scenario, or even know how you want to handle this scenario, so its choice would amount to terminating the thread and leaving your program in a strange state. This could lead to resource leaks, hangs, corruption of data, etc. By terminating the process given an unhandled exception you know exactly what happens, the process ends.
This is a normal behavior of the CLR from v2.0. Here is a MSDN post on this. To avoid process from terminating you could use something like this
<legacyUnhandledExceptionPolicy enabled="1"/>
which is not advisable.
It is normal behavior. Perhaps you want to catch the exception to prevent the application from exiting.
i want to know do finally block still execute in exception handling even if there is no matching catch block for the try block and if not then what happens?
Also i want to now system exception and application difference
Yes, you do not need a catch block at all. The finally block always executes.
As to the difference between System.Exception and System.ApplicationException: Exception is the base class for all exceptions; ApplicationException should be used when a non-fatal application error occurs. See the MSDN documentation.
Also see best practices for handling exceptions.
As others mentioned finally will run even if there is no catch block. This supports Java's try finally pattern (which can be achieved using IDisposable and using).
One exception (see what I did there?) is when a StackOverflowException is thrown in which case the finally block will not run (nor would a catch if one were present).
The finally block runs after the try block completes (either cleanly or by throwing an exception) as you would expect from its location in the code.
Is there any even small possibility that finally will not be invoked but application still be running?
I'm releasing semaphore there
finally
{
_semParallelUpdates.Release();
}
and afraid of lost of some of them.
Only critical finalizers have a strong guaranty to be called in case the shit hits the fan. You can inherit from CriticalFinalizerObject or SafeHandle to get this behavior. However, letting your finalizer do anything else than call code with a strong reliability contract is not advised. This code has to be able to run for instance, in situations where the system is out of memory.
Implementing a critical finalizer is only needed in situations where you want to be sure all unhandled resources are cleaned even if the app domain is unloaded (because for instance). Please note that you of course never have any guarantee that finalizers will run. In case of a power failure, you're out of luck. If you need more guarantees, you need some sort of transactional system (such as a database) that can get you this behavior.
If you are looking for ways to make your code reliable, I suggest reading the following article:
Reliability Best Practices
Another excellent article recommended by VinayC is the following:
Stephen Toub: Keep Your Code Running with the Reliability Features of the .NET Framework
Host windows operating system will terminate the program even in worst cases of uncaught fatal exceptions. But yes even in that case finally block will surely get executed.
In Framework 1.0 and 1.1, this was possible if a thread which is currently in the finally block was aborted using Thread.Abort. In current versions of the framework, I'm not aware of any such cases.
Is it guaranteed that your code will reach the finally? Yes (barring some cataclysmic event such as the world coming to an end... or, you know, your computer losing power or your OS crashing).
But it's important to recognize that, if it's absolutely critical that your code runs, you better make sure your code doesn't throw an exception itself!
Take this for example:
IDisposable someDisposableObject = null;
IDisposable someOtherDisposableObject = null;
try
{
someDisposableObject = GetDisposableObject();
throw new Exception("Holy crap, something bad happened.");
someOtherDisposableObject = GetOtherDisposableObject();
}
finally
{
// This will throw a NullReferenceException...
someOtherDisposableObject.Dispose();
// ...so this actually won't run.
someDisposableObject.Dispose();
}
So if you want your entire finally block to be run, it's important to write it properly so that an exception is (ideally) impossible.
The current thread will not leave the current stack frame unless or until the "finally" block executes or an exception is thrown from within the finally block itself. If a thread dies or gets blocked within a "try" block, execution will never leave the current stack frame, but it won't execute the finally "block" either.