What happens in a C# program if an exception is not caught. Does the program 'crash' with something like a run-time error?
What happens in a C# program if an exception is not caught. Does the program 'crash' with something like a run-time error?
Maybe.
Maybe not.
What happens is implementation-defined.
For example, you might get a dialog box that says "there was an unhandled exception, and I see you have Visual Studio installed. Do you want to start up the debugger and examine the program state?"
You might get a dialog box that says "there was an unhandled exception, do you want to report this to Microsoft?"
If you are already running in the debugger, the debugger probably does something to bring it to your attention.
The runtime is allowed to do whatever it wants, and that includes asking you what to do.
Note that the runtime is aware of whether there's going to be a catch block or not before the finally blocks run. You can easily demonstrate this with a console app. Write an app that crashes, and outputs in the finally block:
Unhandled Exception: System.Exception: Exception of type
'System.Exception' was thrown at
ConsoleApplication1.Program.Main(String[] args)
finally running now
See what happens? The runtime reports the error, gives the debugger a chance to run, or reports the problem to Microsoft, or whatever, before it runs the finally blocks. If they run at all. They might not. Anything can happen. The user could decide to destroy the process, or start a debugger and fix the exception, or whatever.
If you really want to understand how exceptions work in C# you should read this:
http://blogs.msdn.com/b/cbrumme/archive/2003/10/01/51524.aspx
Yes.
Yes.
Something "exceptional" has happened, and your program does not know how to handle it, so it must stop execution at that point and "crash". There will be code that is executed after the crash, such as finally blocks, but basically the party is over for your code.
The best thing to do is to log these events, giving as much intofmation about the state of the system/program at the time of the crash. The Logging Application Block is one of the more robust automatic ways to log errors.
Try it! Depending on the error, it will usually catch. Now, as for should all exceptions be caught, if its something like a[i] where it COULD throw an error if i is too big, but you knew that i was supposed to be kept within bounds (for example, in a for loop), you would not catch that exception.
However, if you are using data coming from a user (say, from a GUI) you would almost always validate it.
Try it yourself!
class Program
{
static void Main(string[] args)
{
int[] arr = new int[1];
arr[5] = 6; //throws an exception; what kind?
}
}
Compile and run this in debug mode for a quick answer to your question.
Or just write some programs. Sooner or later, your code will throw exceptions; it happens to all of us, usually more times than we can count. In console or WinForms applications, an unhandled exception will usually crash the program; in ASP.NET, it will generate an error page, but it won't crash the whole website. You can also write custom code that specifies what to do in case of an unhandled exception, so that your application will fail gracefully.
Related
I'm sorry if this has been asked before, I've been looking around for a few hours now and I haven't been able to gain any insight on my problem. I'm calling Invoke on a MethodInfo I've gotten for a class:
MethodInfo mappedMethod = _boundMethod;
mappedMethod.Invoke(pAction.Target, pParameters);
The problem I'm having, is that if mappedMethod ends up throwing an exception, the debugger will not break at the point of the error as it would with an unhandled exception in normal circumstances. Instead, the exception is passed up to the Invoke method, and it becomes "Exception has been thrown by the target of an invocation."
What I'm wondering, is if there's any way I can get the debugger to break inside the invoked function at the location of the actual problem. I've tried this:
try
{
mappedMethod.Invoke(pAction.Target, pParameters);
}
catch (TargetInvocationException e)
{
throw e.InnerException;
}
Which at least throws the actual exception, but the debugger breaks right at the throw and still will not move the debugger cursor to where the error occurred.
I'm aware that you can make the debugger break on all thrown exceptions immediately with Debug->Exceptions, but that's not appropriate in my situation because I'd have to turn it on and off constantly (My code has try-catches everywhere that throw exceptions for user created content fairly often) and above that, the main purpose of my code is to function as a scripting language for 3rd party users. I'm using Invoke to call user scripts, so it's important that my scripting layer is able to show users where their errors occurred without them needing to change debugger settings.
Is there anything I can do, or is Invoke just not going to be able to work in the way I want it to? Again sorry if this has been asked, it seems like something somebody else would have wanted to know too!
If a .net program fails to explicitely set the exit code before terminating (by calling Environment.Exit() / Appliation.Current.Shutdown() / ...), what is the exit code for that process?
Does a normal termination always result in exit code zero, and what are the other possible cases?
According to this answer to the related question Getting ExitCode From Exception Handler by Hans Passant: "if a program dies on an exception then its exit code is normally the same as the underlying exception error code".
So an uncaugth exception can cange the exit code. Is this always the case, and is the underlying exception error code always guaranteed to be different from zero, and in a specific range?
Are there other circumstances where the .net framework or Windows can automatically set another exit code, such as some non-exception related crash (is that possible?), or a forced task kill?
To put it another way, could I determine by the exit code whether the program terminated in any abnormal fashion or not?
Or if an exit code of zero can happen in some abnormal cases as well, could I include a Environment.Exit(somevalue) in all normal termination paths for a program, and be sure that this exit code can never occur in case of a crash?
Motivation:
Since not all exeptions are catchable without severe workarounds, and since there may be other causes for sudden program termination other than uncaught excpetions, making sure that all code paths call Environment.Exit() is not alwas possible. This is why I am interested in determinining whether the exit code can be used to reliably tell whether a program exited normally.
then its exit code is normally the same as the underlying exception error code
After 6 months, you should have built up some confidence that normally applies. Probably worked just fine, you just can't get a warranty here. It is not just unhandled exceptions that make a process terminate and you can never be sure that it is always the same code that runs when a process dies on an exception.
There is far too much stuff out there that wants to be involved with this and it is not always of the best quality. On top of the list is certainly anti-malware, there is a lot of cr*pware out there and you'll never know what you run into. The recent Avast disaster gives plenty of reasons to be concerned about this. Not nearly where it ends, utilities that replace WER are common enough.
And you are completely defenseless against a yokel that thinks that TerminateProcess() is a good way to solve a file locking problem. Or somebody tripping over the power cord and unplugging the machine.
Focus a bit on why you want to know the exit code. There ought to be something you do next that uses the result of the program to continue executing. Validate the result.
Is the process you're executing your own? That is, can you control its exit code under predictable circumstances?
If so, you could catch any exception thrown and then return your own predictable non-zero exception code (a specific value or your own range.) I would use a negative number since system error codes are positive.
That way you've got three possibilities:
Zero - success
Negative - process threw an exception that you were able to catch
Positive - process threw an exception that you couldn't catch. That would be abnormal.
Anything other than that seems unpredictable. If something is so far outside of normal that a process can't even return a non-zero exit code then the process waiting for that exit code may also have failed. Checking for a non-zero exit code is what you can reasonably do to account for the possibility of failure due to any unforeseen cause.
I don't know if #3 is even possible, but if you're trying to account for the unknown then it's probably the most you can do.
Here's an example:
internal class Program
{
private static void Main(string[] args)
{
var exitCode = 0;
try
{
//do something
}
catch (SystemException systemEx)
{
//log
exitCode = -systemEx.HResult;
}
catch (Exception ex)
{
//log
Environment.ExitCode = int.MinValue;
}
Environment.ExitCode = exitCode;
}
}
That's what you could do. But I wouldn't use the exit code to describe the error. I'd use logging for that. I'd use the exit code to tell the calling app what to do. Zero or non-zero is probably sufficient for that. I've written more complicated return codes that indicate whether the failure was IO-related or SQL-related and in the end I never used any of it. If it fails I look at my log for an error message.
I have a function that looks like this:
try
{
_dbManager.InsertSearch(some data);
}
catch (Exception ex)
{
//do logging
}
_dbManager uses LINQ to insert data to a SQL database. Yesterday, the machine that hosts the database ran out of hard disk space and my program crashed. I got a crash dump which shows that there was a SqlClient.SqlException raised with an exception message reading something like "Database transaction log is full...".
My question is: Why didn't the exception get caught in the catch block above? The weird thing is, when I tried to reproduce the issue, I could get the same exception, but it was caught by the catch block. What could be the issue?
Second, related question: Imagine if we use a third party library and we don't want any exception thrown. We can use try-catch block, but this only works on calling thread. What if the third party starts new thread and an exception is thrown there? Is there a way to handle this? I know I can register our UnhandledExceptionHandler, but that seems to be a different from what I wanted.
My question is: Why didn't the exception get caught in the catch block above?
As David Stratton suggests, the system might have been out of disk space and not able to write to log file. There is also a chance that the process was terminated due to Corrupted State Exception that will not be delivered to your catch-all block in .NET 4. The exception that terminated process might have also been thrown from the thread that did not have catch-all.
Second, related question: Imagine if we use a third party library and
we don't want any exception thrown.
I think that you will have to stop right there and rethink it. What you saying is that you are absolutely 100% sure that nothing ever can go wrong in the thirdparty library. There are certain exceptions (like OutOfMemoryException) that you should not be catching because your code simply does not know how to recover from them. The rule of thumb with exception handling is that you should only catch the exceptions that you fully understand and know how to recover from. Please take a look at this answer and the links in it.
What if the third party starts new thread and an exception is thrown
there? Is there a way to handle this?
The best way to handle this is to rely on default CLR policy that will terminate your application. The only reasonable thing you can do is try to log it by subscribing to AppDomain.UnhandledException.
Is it possible that there is another try/catch block inside that InsertSearch method, which has conditional throw statement.
The implementer of the 'DBManager' class, chose not to throw in case there is not enough space to write on the disk. Just a thought.
If you use Code contracts runtime checking I advice you to check compiled version of your DLL.
Read this for details Why .net exception is not caught?
I have a strange scenario when my application can get into an infinite loop when shutting down. It occurs when it wants to do something but the calls fail as it no longer has access (time based). In such a scenario it should just stop.
I record failed attempts and if the count goes above a certain number in a certain time I throw an exception which I expect to start the JIT debugger and stop the application.
I am not entirely sure why it gets into the loop so I want the JIT window that gives me information like the call stack and application status.
I have exception handling, but what I want is to turn it off and somehow generate an exception that will trigger the JIT debugger, however, all other posts I have found have been for handling exceptions and avoid crashes.
If there is another way at runtime to have the application stop and tell me what is happening I would like to know.
Thanks.
You should try Debugger.Launch() and Debugger.Break() methods (but remember that you should use them only in development environment). You can read more here: http://msdn.microsoft.com/en-us/library/7kzs2ysh.aspx
This line:
System.Diagnostics.Debugger.Launch();
will launch the debugger for you. Give it a try.
I have exception handling
Sounds to me that you have too much of it. Only ever catch specific exception types, never catch Exception. Now you can simply throw any other exception type and your app will bomb with an unhandled exception. Which brings up the JIT debugger dialog on your dev machine, a merciful end on your customer's machine.
Using System.Diagnostics.Debugger is good too, but wrap it with #ifdef DEBUG. Your customer doesn't have one.
I always thought that in Visual Studio and C#, an unhandled exception leads invariably to program termination. Now I know I can continue. Also when the application is running in the debug mode, I can "skip" unhandled exceptions while once the program is deployed (or how is it called when you make an executable?) it will crash it?
I think these articles would be useful for you to read:
Exception Handling (MSDN)
Handling and Throwing Exceptions (MSDN)
It's better to grasp the basics then decide what fits best for you.
catch(Exception ex)
{
#if DEBUG
Console.WriteLn("oops")
#else
throw
#endif
}
The exception means something unexpected occured. If the application doesn't know how to respond, it would crash.
The reason we have the constructs like try catch finally is to make the application aware of the unforeseen issues and how to react in such cases.
If you implement exception handling properly, your application would always run smoothly. It might come to termination, but even that wouldn't be abrupt.