Is it possible to catch an exception from anywhere in a console app that would cause the app to terminate? I'm trying to do this without having and try{} catch{} around everything.
You can use AppDomain.CurrentDomain.UnhandledException event which will catch all unhandled exceptions.
Also check this thread for reference: .NET Global exception handler in console application
Try - catch in your main will not be able to catch exceptions from other threads, for example.
Exceptions "bubble up" to the calling method so having a try-catch block in your Main method is enough to catch everything.
The next important question is what you are going to do with exceptions at top level. Once you are back at that level it is often impossible to make any relevant error recovery except retrying the operation. It is often much better to catch any exceptions that are likely to occur at the site where they are thrown and implement recovery code there.
Go to Debug->Exceptions (Ctrl+D,E) and check appropriate Exception you want to handle. There's an option to break on all 'Thrown' and 'User-unhandled' exception.
You probably want to select 'Common Language Runtime Exception' that are 'Thrown' and 'User-unhandled'. This would break execution any time a CLR exception occurs.
See Most Useful VS Feature No One Knows About
AppDomain.CurrentDomain.UnhandledException += OnCurrentDomain_UnhandledException;
//in windows forms you need also to add these two lines
Application.ThreadException += OnApplication_ThreadException;
Application.SetUnhandledExceptionMode(UnhandledExceptionMode.CatchException);
In a console app you can simply wrap the Main loop in a Try Catch Block. You should know that there are a handful of exceptions that will cause the application to terminate without hitting this block.
Also power failure and setting Environment.FailFast to true will ignore this block.
StackoverflowException msdn docs (You can actually catch it you just shouldn't knowledge here)
Environment.FailFast msdn docs
Check out the AppDomain.CurrentDomain.UnhandledException API for creating an app-global exception handler. And here's a nice blog post about implementing/using it with C# and VB.
Also reconsider your approach to exception handling. Structured exception handling should save you the trouble of needing to deal with 'errors all over the place' because the exceptions percolate up the call stack automatically. This means that if you take advantage of exception handling properly then you will have a small number of points in your code where you handle particular exceptions. You definitely should not try and catch everywhere, all the the time.
A try{} / catch{Exception} in your top-level will catch anything thrown from below before it becomes an "uncaught exception".
Related
I have been asked to add error logging to an application that logs whenever an exception is thrown.
I have a method which will perform the log and I could call this in every Catch clause.
This seems daft as there are literally hundreds of try catch statements in the app.
In visual studio you can set the IDE to break after every exception regardless of try catches, so I am wondering if this sort of functionality is possible to use (albeit not breakpointing the code when thrown)
in summary my question is:
Is there a way to fire an event (or similar) that would call this method, whenever a custom or CLR exception is thrown.
as a bonus question related to this, is it possible to do similar with method entry / exit logging?
C#
.Net 4.5
VS 2012
If you want your function to be called just like how Visual Studio would break on a first chance exception (breaks on a throw even inside a try-catch block) then you need to subscribe to the FirstChanceExecption event in the AppDomain you are running in.
AppDomain.CurrentDomain.FirstChanceException += YourLoggingFunction;
You need to do this for every AppDomain in your program, but unless you are doing non-common tasks 99% of all programs will likely only ever have a single AppDomain
See this MSDN page for more information about subscribing to other AppDomains if you are using them.
Be aware, you may get more exceptions than you realize, some .NET framework code throws exceptions internally and just have the code in try-catch blocks so it never comes up to the surface1 or if the library code has the pattern
try
{
SomeFunctionThatThrows()
}
catch (Exception innerException)
{
throw new SomeMoreDetailedException(message, stateDetails, innerException);
}
subscribing to the event will cause you to start seeing both the exception thrown from SomeFunctionThatThrows() and from throw new SomeMoreDetailedException(2.
1: This is most common in network based I/O calls where an exception like a TimeoutException would be common but the end user does need to know a timeout error happened as there may be things like internal retry logic that happens before the user is notified.
2: You could also see them in Visual Studio by disabling "Just My Code" and breaking on all thrown exceptions.
I'm in the needing of write a custom exception handler...
MY application is probably going to throw different kind of exceptions, and I would like they all get handled by a single handler (so that I don't need to use thousands of "try-catch" blocks).
I tried with the AppDomain's UnhandledException handler, but it seems that, when an exception is caught, the application will inevitably be closed.
So, is there any way I could accomplish my idea?
EDIT:
Thank you for your rapid replies, but I'd like to make you understand better my situation: I have, for example, a class which throws a custom exception (reversible). Now, this class, is called by multiple other class, so I would need to write a try-catch block on every single one of them (or at least this is what your replies make me think about)..
So, here comes the needing of have an handler capable of catch them all...
You should catch specific exceptions at the point in your code where you are best able to handle them. This should not result in thousands of try-catch blocks, rather, you should find localised areas of exception handling.
I tend to handle exceptions at service boundaries, for example, when interacting with databases or file-systems. At these service boundaries you can handle an exception and perform some logical recovery.
If you find yourself writing try-catch blocks where the catch does not add value, does not help your code recover, then you are not adding value by catching the exception!
You shouldn't need 'thousands' of try/catch blocks.
Only catch what you understand at the points where you can make a decision.
Catching an exception is very much part of your logic (unlike try/finally) and using 1 central handler usually won't do.
Consider this as an opportunity to study the proper use of exceptions.
You can use the AppDomain's UnhandledException Handler without closing the application. I do it too to log missed exceptions:
Application.SetUnhandledExceptionMode(UnhandledExceptionMode.CatchException);
Application.ThreadException += new System.Threading.ThreadExceptionEventHandler(Program_UnhandledException);
And
private static void Program_UnhandledException(object sender, System.Threading.ThreadExceptionEventArgs e)
{
//Handle the exception here...
}
Yet I do agree with the other answers, usually you shouldn't need a catch all exception handler. Handling exceptions where they occur should be the preferred action.
Edit: I don't know if it works the same way in console applications too.
Wrap your code in a try and catch block, which catches a Exception. All Exceptions are derived from Exception, so every exception that is thrown and not handled elsewhere is caught here. But remember if an exception is thrown, you deal with a possible invalid state of your application. You can't just always continue after an exception.
Another, rather direct approach is to register to the Application.ThreadException(WinForms) but again, you should not use this if you plan to continue running the application. This is for cases where you want to display a message, maybe a way to send the error report and then close the application.
try
{
// Code that needs exception handling
}
catch (Exception ex)
{
// Handle exception. You may use ex.GetType()
}
If this is not adequate, please explain why in your question.
Im new to .Net and I'm wondering why this code still raises unhandled exception.
try
{
Application.DoEvents();
}
catch
{
}
Does anyone have any idea?
This code is invoked inside an event handler. It throws NullReferenceException. The funny thing is when I try to put a breakpoint right before this code, the exception is not being thrown. The exception occurs only when I let the code run continuously.
Not all exceptions are catchable. You didn't tell us what exception you are seeing, but note that StackOverflowException and ExecutionEngineException can not ever ever ever be caught. I know the former can happen with Application.DoEvents (often in System.Drawing) but I'm not sure about the latter.
Anyway, your code is evil. Don't swallow exceptions. That means you are swallowing bugs.
In general, try to avoid Application.DoEvents. There are really nasty reentrancy issues that can happen.
Attention to use the Application.DoEvents(), most of the time is used for wrong things, like update a progress bar in a time spend process, wich can be done with asynchronous programming.
You can't catch all exceptions, please tell us what exception are raising.
It could also be that when you are running the application without a breakpoint, the DoEvents is allowing another piece of code to execute and that piece of code is throwing the exception.
You should check the exception's StackTrace to see where it points you (or post the stack trace here and we can look at it). If this is the case you might not be seeing the exception when you're using a breakpoint and stepping because the DoEvents and threading don't fire the same ways.
If the exception is being thrown from elsewhere during the DoEvents that would also explain why you are getting an unhandled exception even though the above code is trying to black-hole any exceptions.
We have UnhandledExceptionEventHandler in place and unexpected exceptions were caught by that handler. But why we still see the following screen? I thought if we handled the exception, it will not go up to the OS. If no exception reach the system level, why that screen still show up?
Registering an UnhandledExceptionEventHandler with AppDomain.UnhandledException does not mean that unhandled exceptions become handled. Instead, it is a mechanism to be able to log the exception and relevant program state to aid in later debugging. The exception will remain unhandled and Windows Error Reporting will be invoked.
In reality, when this event is invoked it's "too late" for the the exception to be handled. Assuming you could tell the runtime to continue execution, where would execution unwind to? Not a single frame on the call stack wanted to handle the exception. At best, the executing thread could be terminated; but what if it's on the only foreground thread? Better to propagate the unhandled exception to the operating system's default unhandled exception filter and let it invoke Windows Error Reporting.
Edit with some additional comments:
Now, certain applications you want to design to be crash-resistant, such as long-running service processes. It may make sense to add "catch-all"* exception handlers in some cases, such as a job queue that executes jobs and it doesn't matter if an individual job fails with an unhandled exception; we log the problem and move on to the next job. However, a root catch-all handler in something like Main makes little sense: your entire application is now in an unknown state. You could log the exception and terminate, but you'd be missing out on the benefits of Windows Error Reporting: post-mortem minidumps and an easy button (the "Debug" button on that dialog) to invoke the registered JIT debugger that will take you directly to the problem. For most software, my advice is to simply let your software crash; in-your-face bugs with minidumps are usually some of the easiest to fix.
*Some exceptions are inherently "uncatchable", such as a StackOverflowException. Others, such as an AccessViolationException are catchable, but are inherently indicative of a serious program state incongruity (couldn't read or write from an expected memory location). It is never a good idea to attempt recovery from such exceptions.
Click the Debug button to see where the exception comes from.
If you don't see it immediately, start your application in Visual Studio, go to the Debug,Exceptions dialog, and check all exceptions. Then rerun your application, investigate the code every time the debugger tells you that a first-chance exception has been encountered, and pass the exception to the application if Visual Studio asks you whether to do this.
This should help you finding the source of the problem.
That's because your unhandled exception occurs in the main thread, and there is almost nothing to do about it. Check this article: What!? A .NET Application Can Die?
Almost nothing, yes, because you can still catch that exception on the Application.Run level. At this point your application is dead anyway, but at least you can "avoid windows crash screen" and implement your own crash screen instead:
static void Main()
{
try
{
Application.Run(new Form1());
}
catch (Exception ex)
{
MessageBox.Show("Oops! Can I has " + ex.Message + "?");
}
}
There are cases when unhadled exceptions will not be handled by UnhandledExceptionEventHandler. For example System.Timers.Timer is swallowing exceptions, so they are not propagated to UnhandledExceptionEventHandler.
We have an interface IPoller for which we have various implementations. We have a process that will take an IPoller and start it in a separate thread. I'm trying to come up with a generic way of providing exception handling for any IPollers which don't do it themselves.
My original thinking was to create an implementation of IPoller that would accept an IPoller and just provide some logging functionality. The question I ran into though is how would I provide this error handling? If I have IPoller.Start() which is the target for the Thread is that where the exception will occur? Or is there something on the thread itself I can hook into?
Something like:
Thread thread = new Thread(delegate() {
try
{
MyIPoller.Start();
}
catch(ThreadAbortException)
{
}
catch(Exception ex)
{
//handle
}
finally
{
}
});
This will ensure the exception doesn't make it to the top of the thread.
You should catch the exception at the method you use at the top of the thread, and do the logging from there.
An unhandled exception (at the top of a thread) will (in 2.0 onwards) kill your process. Not good.
i.e. whatever method you pass to Thread.Start (etc) should have a try/catch, and do something useful in the catch (logging, perhaps graceful shutdown, etc).
To achieve this, you could use:
static logging methods
captured variables into the delegate (as an anonymous method)
expose your method on an instance that already knows about the logger
In .NET 4.0+ you should use Tasks instead of threads. Here's a nice article on exception handling in Task Parallel Library
Take a look at AppDomain.UnhandledException, it will help you at least log those exceptions that you are not handling, and in some cases close down "nicely":
This event provides notification of
uncaught exceptions. It allows the
application to log information about
the exception before the system
default handler reports the exception
to the user and terminates the
application. If sufficient information
about the state of the application is
available, other actions may be
undertaken — such as saving program
data for later recovery. Caution is
advised, because program data can
become corrupted when exceptions are
not handled.
Have a look at
Appdomain.FirstChanceException event
It tells you moment any exception occurs and CLR is looking for stack trace. Also event args tell which type of exception. You can consider it as the central place for logging.