Exception Handling

Exception Handling (Task Parallel Library)

.NET Framework (current version)
 

Unhandled exceptions that are thrown by user code that is running inside a task are propagated back to the calling 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, 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 exception 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. You can also handle the original exceptions by using the AggregateException.Handle method.

Even if only one exception is thrown, it is still wrapped in an AggregateException exception, as the following example shows.

Imports System.Threading.Tasks

Module Example
   Public Sub Main()
      Dim task1 = Task.Run(Sub() Throw New CustomException("This exception is expected!"))

      Try
         task1.Wait()
      Catch ae As AggregateException
         For Each ex In ae.InnerExceptions
            ' Handle the custom exception.
            If TypeOf ex Is CustomException Then
               Console.WriteLine(ex.Message)
            ' Rethrow any other exception.
            Else
               Throw
            End If
         Next
      End Try
   End Sub
End Module

Class CustomException : Inherits Exception
   Public Sub New(s As String)
      MyBase.New(s)
   End Sub
End Class
' The example displays the following output:
'       This exception is expected!

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 want to call the Task.Wait or Task(Of TResult).Wait method to wait for a task's completion, you can also retrieve the AggregateException exception from the task's Exception property, as the following example shows. For more information, see the Observing Exceptions By Using the Task.Exception Property section in this topic.

Imports System.Threading.Tasks

Module Example
   Public Sub Main()
      Dim task1 = Task.Run(Sub() Throw New CustomException("This exception is expected!"))

      While Not task1.IsCompleted
      End While

      If task1.Status = TaskStatus.Faulted Then
         For Each ex In task1.Exception.InnerExceptions
            ' Handle the custom exception.
            If TypeOf ex Is CustomException Then
               Console.WriteLine(ex.Message)
            ' Rethrow any other exception.
            Else
               Throw ex
            End If
         Next
      End If
   End Sub
End Module

Class CustomException : Inherits Exception
   Public Sub New(s As String)
      MyBase.New(s)
   End Sub
End Class
' The example displays the following output:
'       This exception is expected!

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, it is possible that a task may continue to process some items after the exception is raised.

System_CAPS_noteNote

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 exception 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 AggregateException exceptions, you can use the Flatten method to remove all the nested AggregateException exceptions, 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.

Imports System.Threading.Tasks

Module Example
   Public Sub Main()
      Dim task1 = Task.Factory.StartNew(Sub()
                                           Dim child1 = Task.Factory.StartNew(Sub()
                                                                                 Dim child2 = Task.Factory.StartNew(Sub()
                                                                                                                       Throw New CustomException("Attached child2 faulted.")
                                                                                                                    End Sub,
                                                                                                                    TaskCreationOptions.AttachedToParent)
                                                                                                                    Throw New CustomException("Attached child1 faulted.")
                                                                              End Sub,
                                                                              TaskCreationOptions.AttachedToParent)
                                        End Sub)

      Try
         task1.Wait()
      Catch ae As AggregateException
         For Each ex In ae.Flatten().InnerExceptions
            If TypeOf ex Is CustomException Then
               Console.WriteLine(ex.Message)
            Else
               Throw
            End If
         Next
      End Try
   End Sub
End Module

Class CustomException : Inherits Exception
   Public Sub New(s As String)
      MyBase.New(s)
   End Sub
End Class
' The example displays the following output:
'       Attached child1 faulted.
'       Attached child2 faulted.

You can also use the AggregateException.Flatten method to rethrow the inner exceptions from multiple AggregateException instances thrown by multiple tasks in a single AggregateException instance, as the following example shows.

Imports System.Collections.Generic
Imports System.IO
Imports System.Threading.Tasks

Module Example
    Public Sub Main()
       Try
          ExecuteTasks()
       Catch ae As AggregateException
          For Each e In ae.InnerExceptions
             Console.WriteLine("{0}:{2}   {1}", e.GetType().Name, e.Message,
                               vbCrLf)
          Next
       End Try
    End Sub

    Sub ExecuteTasks()
        ' Assume this is a user-entered String.
        Dim path = "C:\"
        Dim tasks As New List(Of Task)

        tasks.Add(Task.Run(Function()
                             ' This should throw an UnauthorizedAccessEXception.
                              Return Directory.GetFiles(path, "*.txt",
                                                        SearchOption.AllDirectories)
                           End Function))

        tasks.Add(Task.Run(Function()
                              If path = "C:\" Then
                                 Throw New ArgumentException("The system root is not a valid path.")
                              End If
                              Return { ".txt", ".dll", ".exe", ".bin", ".dat" }
                           End Function))

        tasks.Add(Task.Run(Sub()
                              Throw New NotImplementedException("This operation has not been implemented.")
                           End Sub))

        Try
            Task.WaitAll(tasks.ToArray)
        Catch ae As AggregateException
            Throw ae.Flatten()
        End Try
    End Sub
End Module
' The example displays the following output:
'       UnauthorizedAccessException:
'          Access to the path 'C:\Documents and Settings' is denied.
'       ArgumentException:
'          The system root is not a valid path.
'       NotImplementedException:
'          This operation has not been implemented.

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 calling thread.

Imports System.Threading.Tasks

Module Example
   Public Sub Main()
      Dim task1 = Task.Run(Sub()
                              Dim nestedTask1 = Task.Run(Sub()
                                                            Throw New CustomException("Detached child 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 CustomException Then
                  ' Recover from the exception. Here we just
                  ' print the message for demonstration purposes.
                  Console.WriteLine(ex.Message)
              End If
          Next
      End Try
   End Sub
End Module

Class CustomException : Inherits Exception
   Public Sub New(s As String)
      MyBase.New(s)
   End Sub
End Class
' The example displays the following output:
'       Detached child task faulted.

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 calling 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 the AggregateException.Handle(Func(Of Exception, Boolean)) method, you can examine the exception type, 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 the AggregateException.Handle method returns.

The following example is functionally equivalent to the first example in this topic, which examines each exception in the AggregateException.InnerExceptions collection. Instead, this exception handler calls the AggregateException.Handle method object for each exception, and only rethrows exceptions that are not CustomException instances.

Imports System.Threading.Tasks

Module Example
   Public Sub Main()
      Dim task1 = Task.Run(Sub() Throw New CustomException("This exception is expected!"))

      Try
         task1.Wait()
      Catch ae As AggregateException
         ' Call the Handle method to handle the custom exception,
         ' otherwise rethrow the exception.
         ae.Handle(Function(e)
                      If TypeOf e Is CustomException Then
                         Console.WriteLine(e.Message)
                      End If
                      Return TypeOf e Is CustomException
                   End Function)
      End Try
   End Sub
End Module

Class CustomException : Inherits Exception
   Public Sub New(s As String)
      MyBase.New(s)
   End Sub
End Class
' The example displays the following output:
'       This exception is expected!

The following is a more complete example that uses the AggregateException.Handle method to provide special handling for an UnauthorizedAccessException exception when enumerating files.

Imports System.IO
Imports System.Threading.Tasks

Module Example
    Public Sub Main()
        ' This should throw an UnauthorizedAccessException.
       Try
           Dim files = GetAllFiles("C:\")
           If files IsNot Nothing Then
              For Each file In files
                 Console.WriteLine(file)
              Next
           End If
        Catch ae As AggregateException
           For Each ex In ae.InnerExceptions
               Console.WriteLine("{0}: {1}", ex.GetType().Name, ex.Message)
           Next
        End Try
        Console.WriteLine()

       ' This should throw an ArgumentException.
        Try
           For Each s In GetAllFiles("")
              Console.WriteLine(s)
           Next
        Catch ae As AggregateException
           For Each ex In ae.InnerExceptions
               Console.WriteLine("{0}: {1}", ex.GetType().Name, ex.Message)
           Next
        End Try
        Console.WriteLine()
    End Sub

    Function GetAllFiles(ByVal path As String) As String()
       Dim task1 = Task.Run( Function()
                                Return Directory.GetFiles(path, "*.txt",
                                                          SearchOption.AllDirectories)
                             End Function)
       Try
          Return task1.Result
       Catch ae As AggregateException
          ae.Handle( Function(x)
                        ' Handle an UnauthorizedAccessException
                        If TypeOf x Is UnauthorizedAccessException Then
                            Console.WriteLine("You do not have permission to access all folders in this path.")
                            Console.WriteLine("See your network administrator or try another path.")
                        End If
                        Return TypeOf x Is UnauthorizedAccessException
                     End Function)
       End Try
       Return Array.Empty(Of String)()
    End Function
End Module
' The example displays the following output:
'       You do not have permission to access all folders in this path.
'       See your network administrator or try another path.
'
'       ArgumentException: The path is not of a legal form.

If a task completes in the TaskStatus.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.

Imports System.Threading
Imports System.Threading.Tasks

Module Example
   Public Sub Main()
      Dim task1 = Task.Factory.StartNew(Sub()
                                           Throw New CustomException("task1 faulted.")
                                        End Sub).
                  ContinueWith(Sub(t)
                                  Console.WriteLine("{0}: {1}",
                                                    t.Exception.InnerException.GetType().Name,
                                                    t.Exception.InnerException.Message)
                               End Sub, TaskContinuationOptions.OnlyOnFaulted)

      Thread.Sleep(500)
   End Sub
End Module

Class CustomException : Inherits Exception
   Public Sub New(s As String)
      MyBase.New(s)
   End Sub
End Class
' The example displays output like the following:
'       CustomException: task1 faulted.

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.

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