I have a method that provides an Exception object and I want to log an error if anything happens in the error handler itself. Here is some pseudocode:
public override void OnError(Exception originalException)
{
try
{
// Do work...
}
catch (Exception e)
{
// How do I create this exception without losing the stack
// trace of e and preserving any inner exceptions that were
// in e at the same time including the details of
// originalException?
Exception newException = new Exception(e.Message, originalException);
Logger.Error("An error occurred", newException);
}
}
Basically, I am trying to combine originalException and e above into one Exception message to pass to a logger object. I suppose one option would be to create 2 separate log messages but it's not ideal.
You could use an AggregateException to wrap multiple exceptions:
public override void OnError(Exception originalException)
{
try
{
// Do work...
}
catch (Exception e)
{
var exs = new AggregateException(originalException, e);
Logger.Error("An error occurred", exs);
}
}
EDIT: If Logger doesn't record contents of the InnerException property (or InnerExceptions in this case) then seems the only option is multiple calls to Logger.Error.
Related
In Windows Forms when an exception is thrown from a call to Control.Invoke or Control.BeginInvoke and it is unhandled by any catch block, it can be handled by a control- or application-level handler via the Windows.Forms.Application.ThreadException event. Its handler takes an EventArgs with a property Exception, which is the exception thrown. However, under the hood, Windows.Forms.Control (undesirably) strips off all but the most inner exception, per this answer.
Found this blog post on it as well, but it doesn't suggest any workaround.
(This currently is causing me to get a trivial stack trace in my error logs--a stack trace that tells me the innermost details of , but there is no calling code mentioned and no way to figure out the code location of such an error.)
Is there a workaround? Is there any way to get all the outer exceptions in my application event handler (which is logging unexpected exceptions for troubleshooting)?
The only suggestion I have found seems to be to catch the exception inside the invoked code and stuff some of its info into Exception.Data, perhaps in a new exception--but if I knew the outer code that caused the exception, I could just fix the bug rather than logging it. Instead, how could I do this globally without wrapping a try-catch around every candidate block of code?
This is admittedly a hack, but it's the best solution I was able to come up with which supports both global exception handling in WinForms and all exceptions, even with inner exceptions.
In the Program.cs:
internal static class Program
{
[STAThread]
static void Main()
{
ApplicationConfiguration.Initialize();
AppDomain.CurrentDomain.FirstChanceException += CurrentDomain_FirstChanceException;
Application.ThreadException += Application_ThreadException;
Application.SetUnhandledExceptionMode(UnhandledExceptionMode.CatchException, true);
Application.Run(new MyMainForm());
}
private static void CurrentDomain_FirstChanceException(object sender, FirstChanceExceptionEventArgs e)
{
_outermostExceptionCache.AddException(e.Exception);
}
private static void Application_ThreadException(object sender, ThreadExceptionEventArgs e)
{
Exception exception = null;
if (e?.Exception != null)
exception = _outermostExceptionCache.GetOutermostException(e.Exception);
// Handle exception
}
private static OutermostExceptionCache _outermostExceptionCache = new();
}
And for that you'll need the OutermostExceptionCache class:
public class OutermostExceptionCache
{
public void AddException(Exception ex)
{
if ((ex != null) && (ex is not TargetInvocationException))
{
Exception innermostException = GetInnermostException(ex);
lock (_syncRoot)
{
RemoveOldEntries();
_cache[innermostException] = new CacheEntry(ex);
}
}
}
public Exception GetOutermostException(Exception ex)
{
Exception innermostException = GetInnermostException(ex);
Exception outermostException = null;
lock (_syncRoot)
{
if (_cache.TryGetValue(innermostException, out CacheEntry entry))
{
outermostException = entry.Exception;
_cache.Remove(innermostException);
}
else
{
outermostException = ex;
}
}
return outermostException;
}
private void RemoveOldEntries()
{
DateTime now = DateTime.Now;
foreach (KeyValuePair<Exception, CacheEntry> pair in _cache)
{
TimeSpan timeSinceAdded = now - pair.Value.AddedTime;
if (timeSinceAdded.TotalMinutes > 3)
_cache.Remove(pair.Key);
}
}
private Exception GetInnermostException(Exception ex)
{
return ex.GetBaseException() ?? ex;
}
private readonly object _syncRoot = new();
private readonly Dictionary<Exception, CacheEntry> _cache = new();
private class CacheEntry
{
public CacheEntry(Exception ex)
{
Exception = ex;
AddedTime = DateTime.Now;
}
public Exception Exception { get; }
public DateTime AddedTime { get; }
}
}
The way this works is by watching every exception, as it is thrown, before the runtime even bubbles the exception up to the nearest catch block. Each time an exception is thrown, it is added to a cache, indexed by the innermost (i.e. base) exception. Therefore, when an exception is caught and a new exception is thrown, with the original one as its inner exception, the cache is updated with that outer exception. Then, when Application.ThreadException event handler is provided with the unwrapped, innermost, exception, the handler can look up the outermost one from the cache.
Note: Since even locally-caught exceptions will get added to the cache (and therefore never removed via a call to GetOutermostException), it timestamps each one and automatically ditches any that are older than 3 minutes. That's an arbitrary timeout which can be adjusted as needed. If you make the timeout too short, it could cause problems with debugging since it can cause the exception handling to revert to handling only the innermost exception if you pause the process too long in the debugger (after the exception is thrown but before it is handled).
This question already has answers here:
Why is TargetInvocationException treated as uncaught by the IDE?
(2 answers)
Closed 8 years ago.
When using property reflection to SetValue, the property throws a TargetInvocationException. However, since the call to SetValue is an invocation, the exception is caught and not handled in the property. Is there way to handle the Target Exception in the property and have it ONLY thrown in the main program?
I want this throw to be as if I just made a method call, not an invocation.
Edit for clarification:
The problem I am having is that within the reflect class, I am getting a debug message that says "Exception was unhandled by user code". I have to 'continue' with the debug session and the inner exception is the 'real' exception. Is this just to be expected? I dont want to get warned (and I dont want to hide warnings), I want the code to fix the warning.
public class reflect
{
private int _i;
public int i
{
get { return _i; }
set
{
try{throw new Exception("THROWN");}
catch (Exception ex)
{ // Caught here ex.Message is "THROWN"
throw ex; // Unhandled exception error DONT WANT THIS
}
}
}
}
class Program
{
static void Main(string[] args)
{
reflect r = new reflect();
try
{
r.GetType().GetProperty("i").SetValue(r, 3, null);
}
catch(Exception ex)
{ // Caught here, Message "Exception has been thrown by the target of an invocation"
// InnerMessage "THROWN"
// WANT THIS Exception, but I want the Message to be "THROWN"
}
}
}
You need the InnerException:
catch(Exception ex)
{
if (ex.InnerException != null)
{
Console.WriteLine(ex.InnerException.Message);
}
}
This isn't specific to reflection - it's the general pattern for any exception which was caused by another. (TypeInitializationException for example.)
Sorry, can't comment yet. Two things:
1) why are you first catching ex in your reflection class and then throwing it again? This shouldn't be the problem, though.
2) I think you are getting your exception. Check the "Exception has been thrown"'s inner exception.
I have a member of a library being declared on the main page of my app:
private OpticalReaderLib.OpticalReaderTask _task = new OpticalReaderLib.OpticalReaderTask();
It works fine until I want to navigate back to this page at a different time. It brings up the error "An exception of type 'System.Exception' occurred in OpticalReaderLib.DLL but was not handled in user code".
Does anyone know why this is happening?
Thanks.
System.Exception is the base class for all Exceptions, so this is a very generic error message.
You could try logging more detail (e.g. exception.Message or exception.InnerException) about the exception that is thrown as part of your investigation. (via a try-catch statement).
It looks like you're initialising a field, where is this code being executed?
Update due to comment
As a temporary solution to discover the exception error.
private OpticalReaderLib.OpticalReaderTask _tempTask;
private OpticalReaderLib.OpticalReaderTask _task
{
get
{
//code to debug the error that is occuring
try
{
if (_tempTask == null)
_tempTask = new OpticalReaderLib.OpticalReaderTask();
else
return _tempTask;
}
catch (Exception exception)
{
//Log the exception detail here
}
}
}
protected override void OnBackKeyPress(System.ComponentModel.CancelEventArgs e)
{
if (_task != null)
{
_task.Completed -= OpticalReaderTask_Completed;
_task.Dispose();
_task = null;
}
base.OnBackKeyPress(e);
}
What is the difference between
try { ... }
catch{ throw }
and
try{ ... }
catch(Exception e) {throw new Exception(e.message) }
regardless that the second shows a message.
throw; rethrows the original exception and preserves its original stack trace.
throw ex; throws the original exception but resets the stack trace, destroying all stack trace information until your catch block.
NEVER write throw ex;
throw new Exception(ex.Message); is even worse. It creates a brand new Exception instance, losing the original stack trace of the exception, as well as its type. (eg, IOException).
In addition, some exceptions hold additional information (eg, ArgumentException.ParamName).
throw new Exception(ex.Message); will destroy this information too.
In certain cases, you may want to wrap all exceptions in a custom exception object, so that you can provide additional information about what the code was doing when the exception was thrown.
To do this, define a new class that inherits Exception, add all four exception constructors, and optionally an additional constructor that takes an InnerException as well as additional information, and throw your new exception class, passing ex as the InnerException parameter. By passing the original InnerException, you preserve all of the original exception's properties, including the stack trace.
The first preserves the original stacktrace:
try { ... }
catch
{
// Do something.
throw;
}
The second allows you to change the type of the exception and/or the message and other data:
try { ... } catch (Exception e)
{
throw new BarException("Something broke!");
}
There's also a third way where you pass an inner exception:
try { ... }
catch (FooException e) {
throw new BarException("foo", e);
}
I'd recommend using:
the first if you want to do some cleanup in error situation without destroying information or adding information about the error.
the third if you want to add more information about the error.
the second if you want to hide information (from untrusted users).
One other point that I didn't see anyone make:
If you don't do anything in your catch {} block, having a try...catch is pointless. I see this all the time:
try
{
//Code here
}
catch
{
throw;
}
Or worse:
try
{
//Code here
}
catch(Exception ex)
{
throw ex;
}
Worst yet:
try
{
//Code here
}
catch(Exception ex)
{
throw new System.Exception(ex.Message);
}
Throwing a new Exception blows away the current stack trace.
throw; will retain the original stack trace and is almost always more useful. The exception to that rule is when you want to wrap the Exception in a custom Exception of your own. You should then do:
catch(Exception e)
{
throw new CustomException(customMessage, e);
}
None of the answers here show the difference, which could be helpful for folks struggling to understand the difference. Consider this sample code:
using System;
using System.Collections.Generic;
namespace ExceptionDemo
{
class Program
{
static void Main(string[] args)
{
void fail()
{
(null as string).Trim();
}
void bareThrow()
{
try
{
fail();
}
catch (Exception e)
{
throw;
}
}
void rethrow()
{
try
{
fail();
}
catch (Exception e)
{
throw e;
}
}
void innerThrow()
{
try
{
fail();
}
catch (Exception e)
{
throw new Exception("outer", e);
}
}
var cases = new Dictionary<string, Action>()
{
{ "Bare Throw:", bareThrow },
{ "Rethrow", rethrow },
{ "Inner Throw", innerThrow }
};
foreach (var c in cases)
{
Console.WriteLine(c.Key);
Console.WriteLine(new string('-', 40));
try
{
c.Value();
} catch (Exception e)
{
Console.WriteLine(e.ToString());
}
}
}
}
}
Which generates the following output:
Bare Throw:
----------------------------------------
System.NullReferenceException: Object reference not set to an instance of an object.
at ExceptionDemo.Program.<Main>g__fail|0_0() in C:\...\ExceptionDemo\Program.cs:line 12
at ExceptionDemo.Program.<>c.<Main>g__bareThrow|0_1() in C:\...\ExceptionDemo\Program.cs:line 19
at ExceptionDemo.Program.Main(String[] args) in C:\...\ExceptionDemo\Program.cs:line 64
Rethrow
----------------------------------------
System.NullReferenceException: Object reference not set to an instance of an object.
at ExceptionDemo.Program.<>c.<Main>g__rethrow|0_2() in C:\...\ExceptionDemo\Program.cs:line 35
at ExceptionDemo.Program.Main(String[] args) in C:\...\ExceptionDemo\Program.cs:line 64
Inner Throw
----------------------------------------
System.Exception: outer ---> System.NullReferenceException: Object reference not set to an instance of an object.
at ExceptionDemo.Program.<Main>g__fail|0_0() in C:\...\ExceptionDemo\Program.cs:line 12
at ExceptionDemo.Program.<>c.<Main>g__innerThrow|0_3() in C:\...\ExceptionDemo\Program.cs:line 43
--- End of inner exception stack trace ---
at ExceptionDemo.Program.<>c.<Main>g__innerThrow|0_3() in C:\...\ExceptionDemo\Program.cs:line 47
at ExceptionDemo.Program.Main(String[] args) in C:\...\ExceptionDemo\Program.cs:line 64
The bare throw, as indicated in the previous answers, clearly shows both the original line of code that failed (line 12) as well as the two other points active in the call stack when the exception occurred (lines 19 and 64).
The output of the rethrow case shows why it's a problem. When the exception is rethrown like this the exception won't include the original stack information. Note that only the throw e (line 35) and outermost call stack point (line 64) are included. It would be difficult to track down the fail() method as the source of the problem if you throw exceptions this way.
The last case (innerThrow) is most elaborate and includes more information than either of the above. Since we're instantiating a new exception we get the chance to add contextual information (the "outer" message, here but we can also add to the .Data dictionary on the new exception) as well as preserving all of the information in the original exception (including help links, data dictionary, etc.).
throw rethrows the caught exception, retaining the stack trace, while throw new Exception loses some of the details of the caught exception.
You would normally use throw by itself to log an exception without fully handling it at that point.
BlackWasp has a good article sufficiently titled Throwing Exceptions in C#.
throw is for rethrowing a caught exception. This can be useful if you want to do something with the exception before passing it up the call chain.
Using throw without any arguments preserves the call stack for debugging purposes.
Your second example will reset the exception's stack trace. The first most accurately preserves the origins of the exception.
Also you've unwrapped the original type which is key in knowing what actually went wrong... If the second is required for functionality - e.g., to add extended information or rewrap with a special type such as a custom 'HandleableException' then just be sure that the InnerException property is set too!
Throw;: Rethrow the original exception and keep the exception type.
Throw new exception();: Rethrow the original exception type and reset the exception stack trace
Throw ex;: Reset the exception stack trace and reset the exception type
If you want you can throw a new Exception, with the original one set as an inner exception.
Most important difference is that the second expression erases the type of the exception. And the exception type plays a vital role in catching exceptions:
public void MyMethod ()
{
// both can throw IOException
try { foo(); } catch { throw; }
try { bar(); } catch(E) {throw new Exception(E.message); }
}
(...)
try {
MyMethod ();
} catch (IOException ex) {
Console.WriteLine ("Error with I/O"); // [1]
} catch (Exception ex) {
Console.WriteLine ("Other error"); // [2]
}
If foo() throws an IOException, the [1] catch block will catch the exception. But when bar() throws IOException, it will be converted to plain Exception and won't be caught by the [1] catch block.
throw or throw ex, both are used to throw or rethrow the exception, when you just simply log the error information and don't want to send any information back to the caller you simply log the error in catch and leave.
But in case you want to send some meaningful information about the exception to the caller you use throw or throw ex. Now the difference between throw and throw ex is that throw preserves the stack trace and other information, but throw ex creates a new exception object and hence the original stack trace is lost.
So when should we use throw and throw e? There are still a few situations in which you might want to rethrow an exception like to reset the call stack information.
For example, if the method is in a library and you want to hide the details of the library from the calling code, you don’t necessarily want the call stack to include information about private methods within the library. In that case, you could catch exceptions in the library’s public methods and then rethrow them so that the call stack begins at those public methods.
Here's an interesting question. I have a system that attempts to run some initialization code. If it fails, we call the deinitializer to clean everything up.
Because we call the deinitializer in exception handling, we run the risk that both initialize and deinitialize will fail, and hypothetically, it now seems that we have to throw two exceptions.
It seems pretty unlikely that we will, though. So what happens and what should the code do here?
try { /* init code here */ }
catch (Exception ex)
{
try
{
_DeinitializeEngine();
}
catch (Exception ex2)
{
throw new OCRException("Engine failed to initialize; ALSO failed to deinitialize engine!", ex2);
}
finally
{
throw new OCRException("Engine failed to initialize; failed to initialize license!", ex);
}
}
You shouldn't throw in the Finally block. Instead, use the InnerException to add information in the throw.
Update
What you have to do is to catch and rethrow with the "history" of exception, this is done with InnerException. You can edit it when bulding a new exception. This is a code snippet I just wrote to illustrate the idea that I explain in all the comments below.
static void Main(string[] args)
{
try
{
principalMethod();
}
catch (Exception e)
{
Console.WriteLine("Test : " + e.Message);
}
Console.Read();
}
public static void principalMethod()
{
try
{
throw new Exception("Primary");
}
catch (Exception ex1)
{
try
{
methodThatCanCrash();
}
catch
{
throw new Exception("Cannot deinitialize", ex1);
}
}
}
private static void methodThatCanCrash()
{
throw new NotImplementedException();
}
No need to use double throw with finalize. If you put a break point at the Console.WriteLine(...). You will notice that you have all the exception trace.
If your clean up code is failing and you cannot leave the application in a clean and known state I would let the exception go unhandled (or catch it with the UnhandledException event to log it) then close the application.
Because if you can't handle the first exception, what point is there in catching the second exception?
If I understand your problem correctly, here's what I would have done:
try { /* init code here */ }
catch (Exception ex)
{
// Passing original exception as inner exception
Exception ocrex = new OCRException("Engine failed to initialize", ex);
try
{
_DeinitializeEngine();
}
catch (Exception ex2)
{
// Passing initialization failure as inner exception
ocrex = new OCRException("Failed to deinitialize engine!", ocrex);
}
throw ocrex;
}
You have two possible exception conditions: one in which the first method failed, and one in which both methods failed.
You're already defining your own exception class. So create another (or extend the first) with a RelatedException or PriorException property. When you throw the exception in the second case, save a reference to the first exception in this property.
It's up to the exception handler that catches this exception to figure out what to do with the second exception.