I am new to tasks and I hope you can help me out with this.
Here is code:
Task tast = null;
try
{
tast = new Task(() =>
{
...
});
tast.Start();
if (tast != null)
{
tast.Wait();
if (tast.Exception != null)
{
// catch exception here
}
}
}
catch (Exception err)
{
// not here?
}
The exception is being caught inside catch statement but not inside task.Exception != null.
Why is this happening? Task should be on own thread.
I would rather like to make the task know about exception and then ask if exception != null.
How can I make that work?
I am sorry in case this is a duplicate. Just let me know in comments and I will remove this question.
You're calling Task.Wait() - which will throw an exception if the task is faulted. If you don't call Task.Wait(), you won't get the exception in your thread... but of course you won't spot when it's finished, either. There are various ways you could wait until it's finished (such as attaching a continuation task and waiting until that completes) but the simplest approach is just to call Task.Wait() with a catch block:
try
{
task.Wait();
}
catch (AggregateException)
{
// We'll handle this later.
}
Related
I am following this MSDN guide to handle the exceptions within a Task.
This is what I wrote:
var myTask = Task.Run(() =>
{
throw new Exception("test");
});
try
{
myTask.Wait();
}
catch (Exception e)
{
return false;
}
I have set a breakpoint within the catch block, but at debug runtime, the code does not reach the breakpoint, and it's giving me:
Exception is unhandled by user code
I have no idea what is going on as I have followed very closely to the example from the MSDN guide. In fact, I copied the example to my project and it's still giving the same problem.
Is there any method I can handle the exception outside the Task? I need to return a boolean value based on the fact if the task throws any Exception or not.
Edit
To make it clearer for some of you, this is a more complete set of codes:
public bool ConnectToService()
{
try
{
// Codes for ServiceHost etc etc, which I'm skipping
// These codes are already commented out for this test, so they do nothing
var myTask = Task.Run(() =>
{
// Supposed to connect to a WCF service, but just throwing a test exception now to simulate what happens when the service is not running
throw new Exception("test");
});
try
{
myTask.Wait();
}
catch (Exception e)
{
return false;
}
return true;
}
catch (Exception)
{
return false;
}
}
Caller:
public void DoSomething()
{
try
{
// Other irrelevant stuff
if (ConnectToService())
{
DoAnotherThing();
}
}
catch (Exception)
{
}
}
I would also want to point out I have a solution for this, but it's puzzling why an example from MSDN isn't working for me. I would think that my own solution is not elegant, so I'm still looking for a more elegant solution.
Exception taskException = null;
var myTask = Task.Run(() =>
{
try
{
throw new Exception("test");
}
catch (Exception e)
{
taskException = e;
}
});
try
{
myTask.Wait();
if (taskException != null) throw taskException;
}
catch (Exception e)
{
return false;
}
When a task is run, any exceptions that it throws are retained and re-thrown when something waits for the task's result or for the task to complete
task.Wait() Rethrows any exceptions
task.Result Rethrows any exceptions
As well, your code works correctly
Just press f5 while catching an exception and you will see that will get your point
According to MSDN Task.Run:
Queues the specified work to run on the thread pool and returns a Task object that represents that work.
So You throwing your exception on different thread than you trying to catch it. You should deal with exception on same thread.
Alternatively you can deal with unhandled exceptions in global AppDomain.UnhandledException event.
Jai, as mentioned, this code will always work. I think you will have to enable some settings in visual studio. The setting is turned off and because of this, you are getting "Exception not handled by user code".
try checking Under Tools, Options, Debugging, General, Enable just my code.
Also, you can use something like below if you don't like to bother about try/catch stuff :
myTask.ContinueWith(<you can access Exception property here to see if there was an exception>)
I had the same Problem and solved with ContinueWith
See:
var task = Task.Run(() =>
{
ChatHubWrapper chatHub = Ordem_ServicoBLL.sendMensagemIniciarChatPelaVr(pessoaWrapper.OrdemServico);
foreach (var mensagem in chatHub.MensagensEnviadas)
ChatHub.sendMensagemTodaSala(pessoaWrapper.OrdemServico.ID, mensagem);
})
.ContinueWith((t) =>
{
if (t.IsFaulted)
setPanelErrorWhats(t.Exception.InnerException.Message); // or throw new Exception...
});
task.Wait();
if (task.IsCompleted)
Response.Redirect(pessoaWrapper.OrdemServico.getUrlViewOSSuporte());
With this you Don't need a create Exception taskException = null;
And is not good to use catch Inside Task.Run
#Jai, please try to move a Task.Run to the inside of try/catch block. I think Task.Run executes imediatelly so you may get exception because of that.
In the method below, when an exception is thrown in the TRY block, it is being swallowed. How can I make it throw the exception so that it gets written to log in the catch block? The log writer works fine. Thanks!
public static bool MonitorQueueEmptyTask(string queueName, CancellationTokenSource tokenSource)
{
try
{
Task<bool> task = Task.Factory.StartNew<bool>(() =>
{
while (!QueueManager.IsQueueEmpty(queueName))
{
if (tokenSource.IsCancellationRequested)
{
break;
}
Thread.Sleep(5000);
throw new Exception("Throwing an error!"); //THIS THROW IS SWALLOWED -- NO LOG WRITTEN ON CATCH
};
return true;
}, tokenSource.Token);
}
catch (Exception ex)
{
WriteExceptionToLog(ex.Stack); //it's not that this method doesn't work. it works fine.
return false;
}
return true;
}
If you want to fire and forget, you can attach a continuation using ContinueWith. The current try-catch will not help you at all, as the exception is encapsulated inside the Task. If this is "fire and forget", than you can log the exception:
public static Task MonitorQueueEmptyTask(
string queueName, CancellationTokenSource tokenSource)
{
return Task.Factory.StartNew<bool>(() =>
{
while (!QueueManager.IsQueueEmpty(queueName))
{
if (tokenSource.IsCancellationRequested)
{
break;
}
Thread.Sleep(5000);
throw new Exception("Throwing an error!");
};
}, tokenSource.Token, TaskCreationOptions.LongRunning).ContinueWith(faultedTask =>
{
WriteExceptionToLog(faultedTask.Exception);
}, TaskContinuationOptions.OnlyOnFaulted);
}
This, in turn, will not propagate the exception after it's thrown, but will provide a mechanism to log the error. If you want the exception to be properly handled, you can register to TaskScheduler.UnobservedTaskException. Additionally, you can set ThrowUnobservedTaskExceptions enabled="true" in your configuration if you want unhandled exceptions to terminate your application. ContinueWith will consider the exception "handled" once you look at the task.Exception property.
The exception is not swallowed; it's just that it doesn't occur on the thread that executes the try/catch block, but on the separate Task thread.
If you don't observe the task's result or exception, when the task is eventually garbage collected, it will throw an exception saying that the task was not observed. Unless you catch that by handling the TaskScheduler.UnobservedTaskException, it will crash the process.
I also had a problem with this, and i really dislike the whole idea of App.config, so can provide another solution to prevent the exceptions disappearing :)
Save the exception then throw it after the Task.Run has completed, e.g.
private async void Function() {
Exception save_exception = null;
await Task.Run(() => {
try {
// Do Stuff
} catch (Exception ex) {
save_exception = ex;
}
}).ContinueWith(new Action<Task>(task => {
if (save_exception != null)
throw save_exception;
// Do Stuff
}));
}
I have a task running a long time operation in WPF:
Task t = Task.Factory.StartNew(() =>
{
try
{
process(cancelTokenSource.Token, CompressionMethod, OpInfo);
}
catch (OperationCanceledException)
{
logger.Info("Operation cancelled by the user");
}
}, cancelTokenSource.Token);
try
{
t.Wait();
}
catch (AggregateException ae)
{
int i = 0;
}
private void process(CancellationToken token, CompressionLevel level, OperationInfo info)
{
// check hash
if (ComputeHash)
{
logger.Info("HASH CHECKING NOT IMPLEMENTED YET!");
MessageBox.Show(this,"HASH CHECKING NOT IMPLEMENTED YET!", "WARNING", MessageBoxButton.OK, MessageBoxImage.Warning);
}
token.ThrowIfCancellationRequested();
UserMsgPhase = "Operation finished";
return info;
}
Problem is "MessageBox.Show" throws an exception and it is not captured within "catch (AggregateException ae)". I've been reading about TPL exception handling but I don't understand why it is not catched. Please, could you help me?
Once the task is complete you can check its Exception property. You also have Status and IsCompleted properties which may be useful to you...
Check Task.Exception.
If your task is typed (returning a result), then accessing myTask.Result will throw this exception.
Moreover, if you are running .Net 4.5, you could use async/await.
As an example:
public async void MyButton_OnClick(object sender, EventArgs e)
{
try
{
Task t = ...your task...;
var myResult = await t; // do whatever you like with your task's result (if any)
}catch
{
// whatever you need
}
}
as you would do with synchronous code (but this is not an actual synchronous call)
I believe that the question's process method is a Task, so it looks like it could be implement in a different manner:
You can make the process to be implemented as Task and then you will have a task-child within task-parent.
Then you can make use of the TaskCreationOptions.AttachedToParent option.
According to Stephen Toub, using AttachedToParent will help notify children-task exception to the parent-task catch:
any exceptions from faulted children will propagate up to the parent
Task (unless the parent Task observes those exceptions before it
completes).
Example:
I've omitted the cancellation token parts in order for it to be more simple.
Task t = Task.Factory.StartNew(() =>
{
var process = new Task(() =>
{
//Copy here the process logic.
}, TaskCreationOptions.AttachedToParent);
//*Private failure handler*.
process.start();
});
try
{
t.Wait();
}
catch (AggregateException ae)
{
//handle exceptions from process.
}
In addition, you may add a private failure handler like:
//*Private failure handler*.
var failHandler = child.ContinueWith(t =>
{
//Oops, something went wrong...
}, TaskContinuationOptions.AttachedToParent|TaskContinuationOptions.OnlyOnFaulted);
i am using the following
Task.Factory.StartNew(() => DoPrintConfigPage(serial));
then the function i am calling looks like this
private void DoPrintConfigPage(string serial)
{
//do printing work
}
My problem is an exception is being thrown inside the thread and not being handled.
I have tried wrapping it in a try catch
try
{
Task.Factory.StartNew(() => DoPrintConfigPage(serial));
}
catch (Exception ex) { }
but it still is not catching the error and thus crashing the application.
How can I catch exceptions in the main thread so I can handle them?
Update
I have made the changes recommended below and still it is saying the exception is unhandled
var task = Task.Factory.StartNew(() => DoPrintConfigPage(serial))
.ContinueWith(tsk =>
{
MessageBox.Show("something broke");
},TaskContinuationOptions.OnlyOnFaulted);
then in my DoConfigPage I added another try catch.
In this catch is now where it is crashing and saying the exception being thrown was unhandled, what am I missing?
private void DoPrintConfigPage(string serial)
{
try
{
//call the print function
}
catch (Exception ex)
{
throw ex; //it is crashing here and saying it is unhandled
}
}
I also tried what Eric J. suggested with the same results
var task = Task.Factory.StartNew(() => DoPrintConfigPage(serial));
try
{
task.Wait();
}
catch (AggregateException ex) { MessageBox.Show("something broke"); }
Alternatively, you can chain your task creation and add a ContinueWith:
var job = Task.Factory
.StartNew(...)
.ContinueWith(tsk =>
{
// check tsk for exception and handle
});
EDIT: This snippet, when run, pops up the message box for me:
void Main()
{
var serial = "some serial";
var task = Task.Factory
.StartNew(() => DoPrintConfigPage(serial))
.ContinueWith(tsk =>
{
MessageBox.Show("something broke");
var flattened = tsk.Exception.Flatten();
// NOTE: Don't actually handle exceptions this way, m'kay?
flattened.Handle(ex => { MessageBox.Show("Error:" + ex.Message); return true;});
},TaskContinuationOptions.OnlyOnFaulted);
}
public void DoPrintConfigPage(string serial)
{
throw new Exception("BOOM!");
}
Your try block is exited right after you start the new task, because that method just continues to run.
Instead you can catch the Exception as an AggregateException where you wait for the task (or multiple tasks) to complete:
var task1 = Task.Factory.StartNew(() =>
{
throw new MyCustomException("I'm bad, but not too bad!");
});
try
{
task1.Wait();
}
catch (AggregateException ae)
{
// Assume we know what's going on with this particular exception.
// Rethrow anything else. AggregateException.Handle provides
// another way to express this. See later example.
foreach (var e in ae.InnerExceptions)
{
if (e is MyCustomException)
{
Console.WriteLine(e.Message);
}
else
{
throw;
}
}
}
http://msdn.microsoft.com/en-us/library/dd997415.aspx
If you are not waiting on your task, I think the easiest solution is found in Task.Exception:
Gets the AggregateException that caused the Task to end prematurely.
If the Task completed successfully or has not yet thrown any
exceptions, this will return null.
I am using something like this:
Task.Factory.StartNew(() => DoStuffHere())
.ContinueWith(task =>
{
if (task.Exception != null)
Log("log all the exceptions!");
});
You should also know about
System.Threading.Tasks.TaskScheduler.UnobservedTaskException.
If you are in the business of creating "fire and forget" Task instances, you'll want to subscribe to that event at the start of your program.
Maybe you are trying to catch a Corrupted State Exception. Since .NET 4 applications are unable to catch such exceptions by default. You could try to add the legacyCorruptedStateExceptionsPolicy=true entry to your configuration file as stated in the MSDN article linked above.
I have this simple code :
var g= Task.Factory.StartNew<int> (() => 8)
.ContinueWith (ant =>{throw null;})
.ContinueWith (a =>{ Console.WriteLine("OK");},TaskContinuationOptions.NotOnFaulted);
try{
Console.WriteLine("1");
g.Wait();
Console.WriteLine("2");
}
catch (AggregateException ex)
{Console.WriteLine("catch"); }
The Output :
1
catch
System.AggregateException: A Task's exception(s) were not observed either by Waiting on the Task or accessing its Exception property. As a result, the unobserved exception was rethrown by the finalizer thread.
msdn :
TaskContinuationOptions.NotOnFaulted
Specifies that the continuation task should not be scheduled if its
antecedent threw an unhandled exception. This option is not valid for
multi-task continuations.
ok .
And it is ok - not showing this line cause the prev line DID throw exception.
Questions :
Do I get the AggregateException exception because I haven't inspected the Exception property ?
Must I always inspect if the antecedent throw an exception ( in each line ? ) ? ( I can't check each line ! it doesn't make any sense and very annoying)
Wasn't the try catch block should have swallow the exception ? ( I thought that all exceptions bubble up to the wait method....so ? )
Do I get the AggregateException exception because I haven't inspected
the Exception property ?
No, you get an exception, because task g cancels by TPL(because, as msdn stated, this task will not scheduled if antescendent task throws an exception).
We have 3 tasks here:
Original Task (that uses StartNew)
First Continuation Task (that throws an exception)
Second Continuation Task (that prints OK) (this is g task from your code).
The issue is that you ask TPL to start 3d task only if 2nd task will finished successfully. This means that if this condition will not met TPL will cancel your newly created task entirely.
You got unobserved task exception because you have temporary task (task 2 in my list) that you never observe. An because you never observe it faulted state it will throw in finalizer to tell you about it.
You can check this by printing task's status in catch block:
catch (AggregateException ex)
{
Console.WriteLine("catch");
// Will print: Status in catch: Canceled
Console.WriteLine("Status in catch: {0}", g.Status);
}
Must I always inspect if the antecedent throw an exception ( in each
line ? ) ? ( I can't check each line ! it doesn't make any sense and
very annoying)
Yes you should observe antecedent tasks exception to avoid this issue:
static class TaskEx
{
public static Task ObserverExceptions(this Task task)
{
task.ContinueWith(t => { var ignore = t.Exception; },
TaskContinuationOptions.OnlyOnFaulted);
return task;
}
}
And then use it as following:
var g= Task.Factory.StartNew<int> (() => 8)
.ContinueWith (ant =>{throw null;})
.ObserveExceptions()
.ContinueWith (a =>{ Console.WriteLine("OK");});
try{
Console.WriteLine("1");
g.Wait();
Console.WriteLine("2");
}
catch (AggregateException ex)
{Console.WriteLine("catch"); }
UPDATE: Added solution to last bullet
Wasn't the try catch block should have swallow the exception ? ( I
thought that all exceptions bubble up to the wait method....so ? )
We have set of extension method (called TransformWith) in our project that can solve this particular issue and gain following:
Exception would bubble up to the catch block and
We'll not crash application with TaskUnobservedException
Here the usage
var g = Task.Factory.StartNew(() => 8)
.ContinueWith(ant => { throw null; })
// Using our extension method instead of simple ContinueWith
.TransformWith(t => Console.WriteLine("OK"));
try
{
Console.WriteLine("1");
// Will fail with NullReferenceException (inside AggregateExcpetion)
g.Wait();
Console.WriteLine("2");
}
catch (AggregateException ex)
{
// ex.InnerException is a NullReferenceException
Console.WriteLine(ex.InnerException);
}
And here is a extension method:
static class TaskEx
{
public static Task TransformWith(this Task future, Action<Task> continuation)
{
var tcs = new TaskCompletionSource<object>();
future
.ContinueWith(t =>
{
if (t.IsCanceled)
{
tcs.SetCanceled();
}
else if (t.IsFaulted)
{
tcs.SetException(t.Exception.InnerExceptions);
}
else
{
try
{
continuation(future);
tcs.SetResult(null);
}
catch (Exception e)
{
tcs.SetException(e);
}
}
}, TaskContinuationOptions.ExecuteSynchronously);
return tcs.Task;
}
}
Do I get the AggregateException exception because I haven't inspected
the Exception property ?
Tasks always throw AggregateException : http://msdn.microsoft.com/en-us/library/system.threading.tasks.task.exception.aspx
You can get the original exception using :
var myTask = Task.Factory.StartNew(() => { throw new NotImplementedException(); });
var myException = myTask.Exception.Flatten().InnerException as NotImplementedException;
Must I always inspect if the antecedent throw an exception ( in each
line ? ) ? ( I can't check each line ! it doesn't make any sense and
very annoying)
Yes it is anoying, you should create two continuations for each task to check exceptions : one that checks if there has been an exception to handle it, and another one to continue the operation if there was no exception see TaskContinuationOptions.OnlyOnFaulted and TaskContinuationOptions.OnlyOnRanToCompletion.
You should even create a third continuation to deal with cancellation if needed.
Wasn't the try catch block should have swallow the exception ? ( I
thought that all exceptions bubble up to the wait method....so ? )
No it won't, exceptions are not thrown at higher level, you should use TaskContinuationOptions.OnlyOnFaulted on the task continuation to check if there was an exception. You can get tasks exceptions at caller's level only with the async keyword not available in .net 4
Handle AggregateExceptions like this:
catch(AggregateException aex)
{
aex.Handle(ex =>
{
// Do some handling and logging
return true;
}
}