I just came across odd behavior with exception handling in .Net. (I'm using C# in MS Visual Studio 2008, but one question I saw here seemed to imply that what I see is true throughout the .Net world.) I am writing a plain WinForm application. I am intentionally causing an unhandled exception to be thrown inside a form_load event handler, outside of any try block. I get no notification. If an unhandled exception occurs in a normal method, a message pops up telling me that the exception happened, and giving me some information about the problem. But in the handler, the code just quietly exits the function without letting anybody know that it happened. If I add a try/catch block, the exception is caught as expected.
Is it true that this behavior happens in all event handlers? And is this expected behavior? And if so, is it because there is too much danger of bad things happening if an event handler unexpectedly stops?
Whether inside or outside VS, this behavior occurs when there is a debugger attached to the process. However, being a debug version makes no difference. If running outside VS without a debugger attached, the unhandled exception will fire up.
You can check
Why the form load can't catch exception? , and
VS2010 does not show unhandled exception message in a WinForms Application on a 64-bit version of Windows , for possible solutions.
EDIT: This behavior is only specific to the form_load event handler, as far as I know.
Related
Using UI Automation for some Windows I get the following exceptions on a IUIAutomationElement::FindAll() call using VS2017. First question, what is a wil:ResultException and what does it mean it rethrow at memory address 0? I check the FindAll() result and doesn't seem to have FAILED(hr) because it outputs a debug message if it did and it's not.
Exception thrown at 0x00007FF897AC3E49 in app.exe: Microsoft C++ exception: wil::ResultException at memory location 0x000000550AF2BDC0.
Exception thrown at 0x00007FF897AC3E49 in app.exe: Microsoft C++ exception: [rethrow] at memory location 0x0000000000000000.
I don't know if it is related or not. I turned on the fairly new "Use Text cursor indicator" as users are telling us our app can crash when it is on. After doing some testing, I was closing the app and got an access violation deep in UIAutomation.dll. The system was just exiting the process. I was trying to duplicate the crash and though I didn't crash, I just saw this same message and the reply to this post that mentioned UIAutomation. The "fairly" new setting is new to me because our IT just allowed the version update that has the setting on our boxes.
The crash occurred while doing a PeekMessage during the exit of the process. We had a static c++ object that was being deleted and during that call, it called the API. I rearranged the code to make the call that used PeekMessage so it happened before the process exited. That avoided that crash. However, we are MFC based and in a debug build, if any code does an ASSERT during shutdown, MFC's assert code does a PeekMessage to remove WM_QUIT before showing the assert message box. So, we can still crash there randomly in our debug builds.
When running with the Text Cursor Indicator on, I see a lot of these "wil" exceptions in the debug output window (release or debug builds). Many seem to occur when a window that has the indicator drawn over it closes. Example - standard file open dialog. I open it, click the path edit box and when I close the dialog, I get some of those exceptions. Turning off the indicator setting avoids all of that and the crashes.
My vs2010 shows this message when an exception is thrown:
How can I configure the debugger to show exceptions like this instead:
Thanks!
Big difference between the two. You'll only see the first one when you explicitly configure the debugger to stop when an exception is thrown. You do so with Debug + Exceptions, Thrown checkbox. You don't often use this, really only when your code contains too many catch statements that swallow exceptions inappropriately and making the code misbehave. It also has a knack for showing exceptions in code you didn't write. Click the Break button to allow the debugger to show you the details.
You'll only get the second screenshot when an exception is thrown and there is no catch block to catch it, making it an unhandled exception. That's a fatal error, the program cannot continue. The debugger stops to show you the problem, it is otherwise the end of the debugging session. Without a debugger your program will crash. This is the 'good' kind, you get it by omitting try/catch blocks so your program terminates when something unexpected happens. You will want to write an event handler for the AppDomain.CurrentDomain.UnhandledException event so the user at least has an idea what went wrong. And you for that matter.
I am debuggin a subtle bug that is thrown from a .net custom component (no source and obfuscated). The component throws a NullReferenceException in its OnPaint() method which makes calls to subscribers, including my code. I am 100% sure that the problem is in my code. The problem is that visual studio debugger does not stop at the point where the exception is thrown, as it should (I enabled NullReferenceException in the Debug/Exceptions dialog).
This is the first time I encountered such a problem in visual studio. Any ideas on identifying offending code, please?
Kemal
Likely the problem is your code isn't throwing the NullReferenceException, however it is probably returning null at some point. You could try adding guard conditions at the end of your functions that ensure you are not returning null, such as an Debug.Assert(retVal != null, "Returned null in function").
If you are sure you own code is throwing a NullReferenceException, make sure you turn on thrown exceptions, rather than user-unhandled ones.
I test the exceptions interception, so, I don't need that Visual Studio breaks on thinkgs like thrown new NullReferenceException("myVar").
I have the following under Debug=>Exceptions
however, VS breaks on the exceptions. What should I do?
PS.
for the application unhandled exception, I "catch" them using the Application.UnhandledException as in the the following:
''' <summary>Occurs when the application encounters an unhandled exception.</summary> '
Private Sub Application_UnhandledException(ByVal sender As Object, ByVal e As Microsoft.VisualBasic.ApplicationServices.UnhandledExceptionEventArgs) Handles Me.UnhandledException
Dim message As String = String.Format("An application UnhandledException were thrown.{1}The application will now terminate.{1}'{0}'{1}{1}StackTrace:{1}{2}", e.Exception.Message, Environment.NewLine, e.Exception.StackTrace)
MessageBox.Show(message)
End Sub
I had same problem when I started using VS2010. I have unit tests, which expect exceptions, and I throw exceptions from my functions. These exceptions are supposed to be handled by the user of my library. In Debug->Exceptions dialog, I unchecked check box under User-Unhandled column for Common Language Runtime Exceptions, and VS stopped breaking on these exceptions. By the way, I don't see second column in the dialog you attached here.
If you throw an exception that is not handled anywhere in your code, Visual Studio is going to break. It doesn't have any other choice: there was an unhandled exception. Outside of Visual Studio, the application would show an error message and inform the user that an unhandled exception occurred.
The options you see in the Debug -> Exceptions dialog only allow you to configure whether Visual Studio breaks on all exceptions, including those that are later handled in your code. These are often referred to as "first-chance" exceptions.
Beyond that, you should never throw a NullReferenceException yourself; this is a runtime exception that is reserved for the runtime framework. Instead, you should throw an ArgumentNullException.
The below method works for me in Visual Studio 2015 (a similar process may work for VS2010).
Taken from the Visual Studio documentation on managing exceptions with the debugger:
In the Exception Settings window, open the context menu by right-clicking in window and then selecting Show Columns. (If you have turned off Just My Code, you will not see this command.)
You should see a second column named Additional Actions. This column displays Continue when unhandled by user code on specific exceptions, meaning that the debugger does not break if that exception is not handled in user code but is handled in external code.
You can change this setting either for a particular exception (select the exception, right-click, and select/deselect Continue when Unhandled in User Code) or for an entire category of exceptions (for example, all the Common Language Runtime exceptions).
In my code I had an error that was catched by following exceptions while program was running. However when I was running program in Visual Studio when the error was happening application was simply exiting without any error (other errors usually bring me to the problematic line).
if (ApplicationDeployment.IsNetworkDeployed) {
AppDomain.CurrentDomain.UnhandledException += currentDomainUnhandledException;
Application.ThreadException += applicationThreadException;
}
Of course if i remove the if i get this exception handling done by my methods which simply uses MessageBox to show the error. Is there a way to force Visual Studio to catch this error like it catches other types of errors?
Only by using Debug + Exceptions, Thrown checkbox. That makes the debugger stop on the "first chance". At the point the exception is thrown. You typically want to do this:
if (!System.Diagnostics.Debugger.IsAttached) {
// Subscribe the events
//...
}
Note that this already works that way for Application.ThreadException, Winforms already avoids catching exceptions if it sees a debugger. For the exact same reason.