Exception Handling (Task Parallel Library)
Unhandled exceptions that are thrown by user code that is running inside a task are propagated back to the joining thread, except in certain scenarios that are described later in this topic. Exceptions are propagated when you use one of the static or instance Task.Wait or Task(Of TResult).Wait methods, and you handle them by enclosing the call in a try-catch statement. If a task is the parent of attached child tasks, or if you are waiting on multiple tasks, then multiple exceptions could be thrown. To propagate all the exceptions back to the calling thread, the Task infrastructure wraps them in an AggregateException instance. The AggregateException has an InnerExceptions property that can be enumerated to examine all the original exceptions that were thrown, and handle (or not handle) each one individually. Even if only one exception is thrown, it is still wrapped in an AggregateException.
Dim task1 = Task.Factory.StartNew(Sub() Throw New MyCustomException("I'm bad, but not too bad!") End Sub) Try task1.Wait() Catch ae As AggregateException ' 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. For Each ex In ae.InnerExceptions If TypeOf (ex) Is MyCustomException Then Console.WriteLine(ex.Message) Else Throw End If Next End Try
You could avoid an unhandled exception by just catching the AggregateException and not observing any of the inner exceptions. However, we recommend that you do not do this because it is analogous to catching the base Exception type in non-parallel scenarios. To catch an exception without taking specific actions to recover from it can leave your program in an indeterminate state.
If you do not wait on a task that propagates an exception, or access its Exception property, the exception is escalated according to the .NET exception policy when the task is garbage-collected.
When exceptions are allowed to bubble up back to the joining thread, then it is possible that a task may continue to process some items after the exception is raised.
When "Just My Code" is enabled, Visual Studio in some cases will break on the line that throws the exception and display an error message that says "exception not handled by user code." This error is benign. You can press F5 to continue and see the exception-handling behavior that is demonstrated in these examples. To prevent Visual Studio from breaking on the first error, just uncheck the Enable Just My Code checkbox under Tools, Options, Debugging, General.
If a task has an attached child task that throws an exception, that exception is wrapped in an AggregateException before it is propagated to the parent task, which wraps that exception in its own AggregateException before it propagates it back to the calling thread. In such cases, the InnerExceptions property of the AggregateException that is caught at the Task.Wait or Task(Of TResult).Wait or WaitAny or WaitAll method contains one or more AggregateException instances, not the original exceptions that caused the fault. To avoid having to iterate over nested AggregateExceptions, you can use the Flatten method to remove all the nested AggregateExceptions, so that the AggregateException.InnerExceptions property contains the original exceptions. In the following example, nested AggregateException instances are flattened and handled in just one loop.
' task1 will throw an AE inside an AE inside an AE Dim task1 = Task.Factory.StartNew(Sub() Dim child1 = Task.Factory.StartNew(Sub() Dim child2 = Task.Factory.StartNew(Sub() Throw New MyCustomException("Attached child2 faulted.") End Sub, TaskCreationOptions.AttachedToParent) End Sub, TaskCreationOptions.AttachedToParent) ' Uncomment this line to see the exception rethrown. ' throw new MyCustomException("Attached child1 faulted.") End Sub) Try task1.Wait() Catch ae As AggregateException For Each ex In ae.Flatten().InnerExceptions If TypeOf (ex) Is MyCustomException Then Console.WriteLine(ex.Message) Else Throw End If Next 'or like this: ' ae.Flatten().Handle(Function(e) ' Return TypeOf (e) Is MyCustomException ' End Function) End Try
By default, child tasks are created as detached. Exceptions thrown from detached tasks must be handled or rethrown in the immediate parent task; they are not propagated back to the calling thread in the same way as attached child tasks propagated back. The topmost parent can manually rethrow an exception from a detached child to cause it to be wrapped in an AggregateException and propagated back to the joining thread.
Dim task1 = Task.Factory.StartNew(Sub() Dim nestedTask1 = Task.Factory.StartNew(Sub() Throw New MyCustomException("Nested task faulted.") End Sub) ' Here the exception will be escalated back to joining thread. ' We could use try/catch here to prevent that. nestedTask1.Wait() End Sub) Try task1.Wait() Catch ae As AggregateException For Each ex In ae.Flatten().InnerExceptions If TypeOf (ex) Is MyCustomException Then ' Recover from the exception. Here we just ' print the message for demonstration purposes. Console.WriteLine(ex.Message) End If Next End Try
Even if you use a continuation to observe an exception in a child task, the exception still must be observed by the parent task.
When user code in a task responds to a cancellation request, the correct procedure is to throw an OperationCanceledException passing in the cancellation token on which the request was communicated. Before it attempts to propagate the exception, the task instance compares the token in the exception to the one that was passed to it when it was created. If they are the same, the task propagates a TaskCanceledException wrapped in the AggregateException, and it can be seen when the inner exceptions are examined. However, if the joining thread is not waiting on the task, this specific exception will not be propagated. For more information, see Task Cancellation.
Dim someCondition As Boolean = True Dim tokenSource = New CancellationTokenSource() Dim token = tokenSource.Token Dim task1 = Task.Factory.StartNew(Sub() Dim ct As CancellationToken = token While someCondition = True ' Do some work... Thread.SpinWait(500000) ct.ThrowIfCancellationRequested() End While End Sub, token)
You can use the AggregateException.Handle method to filter out exceptions that you can treat as "handled" without using any further logic. In the user delegate that is supplied to AggregateException.Handle, you can examine the exception type, or its Message property, or any other information about it that will let you determine whether it is benign. Any exceptions for which the delegate returns false are rethrown in a new AggregateException instance immediately after AggregateException.Handle returns.
The following snippet uses a foreach loop over the inner exceptions.
The following snippet shows the functionally equivalent use of the Handle method.
If a task completes in the Faulted state, its Exception property can be examined to discover which specific exception caused the fault. A good way to observe the Exception property is to use a continuation that runs only if the antecedent task faults, as shown in the following example.
In a real application, the continuation delegate could log detailed information about the exception and possibly spawn new tasks to recover from the exception.
In some scenarios, such as when hosting untrusted plug-ins, benign exceptions might be common, and it might be too difficult to manually observe them all. In these cases, you can handle the TaskScheduler.UnobservedTaskException event. The System.Threading.Tasks.UnobservedTaskExceptionEventArgs instance that is passed to your handler can be used to prevent the unobserved exception from being propagated back to the joining thread.