Exception handling (Task Parallel Library)
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 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.
public static partial class Program
{
public static void HandleThree()
{
var task = Task.Run(
() => throw new CustomException("This exception is expected!"));
try
{
task.Wait();
}
catch (AggregateException ae)
{
foreach (var ex in ae.InnerExceptions)
{
// Handle the custom exception.
if (ex is CustomException)
{
Console.WriteLine(ex.Message);
}
// Rethrow any other exception.
else
{
throw ex;
}
}
}
}
}
// The example displays the following output:
// This exception is expected!
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 ex
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 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.
public static partial class Program
{
public static void HandleFour()
{
var task = Task.Run(
() => throw new CustomException("This exception is expected!"));
while (!task.IsCompleted) { }
if (task.Status == TaskStatus.Faulted)
{
foreach (var ex in task.Exception?.InnerExceptions ?? new(Array.Empty<Exception>()))
{
// Handle the custom exception.
if (ex is CustomException)
{
Console.WriteLine(ex.Message);
}
// Rethrow any other exception.
else
{
throw ex;
}
}
}
}
}
// The example displays the following output:
// This exception is expected!
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!
Caution
The preceding example code includes a while loop that polls the task's Task.IsCompleted property to determine when the task has completed. This should never be done in production code as it is very inefficient.
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.
Note
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.
Attached child tasks and nested AggregateExceptions
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, 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.
public static partial class Program
{
public static void FlattenTwo()
{
var task = Task.Factory.StartNew(() =>
{
var child = Task.Factory.StartNew(() =>
{
var grandChild = Task.Factory.StartNew(() =>
{
// This exception is nested inside three AggregateExceptions.
throw new CustomException("Attached child2 faulted.");
}, TaskCreationOptions.AttachedToParent);
// This exception is nested inside two AggregateExceptions.
throw new CustomException("Attached child1 faulted.");
}, TaskCreationOptions.AttachedToParent);
});
try
{
task.Wait();
}
catch (AggregateException ae)
{
foreach (var ex in ae.Flatten().InnerExceptions)
{
if (ex is CustomException)
{
Console.WriteLine(ex.Message);
}
else
{
throw;
}
}
}
}
}
// The example displays the following output:
// Attached child1 faulted.
// Attached child2 faulted.
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.
public static partial class Program
{
public static void TaskExceptionTwo()
{
try
{
ExecuteTasks();
}
catch (AggregateException ae)
{
foreach (var e in ae.InnerExceptions)
{
Console.WriteLine(
"{0}:\n {1}", e.GetType().Name, e.Message);
}
}
}
static void ExecuteTasks()
{
// Assume this is a user-entered String.
string path = @"C:\";
List<Task> tasks = new();
tasks.Add(Task.Run(() =>
{
// This should throw an UnauthorizedAccessException.
return Directory.GetFiles(
path, "*.txt",
SearchOption.AllDirectories);
}));
tasks.Add(Task.Run(() =>
{
if (path == @"C:\")
{
throw new ArgumentException(
"The system root is not a valid path.");
}
return new string[] { ".txt", ".dll", ".exe", ".bin", ".dat" };
}));
tasks.Add(Task.Run(() =>
{
throw new NotImplementedException(
"This operation has not been implemented.");
}));
try
{
Task.WaitAll(tasks.ToArray());
}
catch (AggregateException ae)
{
throw ae.Flatten();
}
}
}
// 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.
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.
Exceptions from detached child tasks
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.
public static partial class Program
{
public static void DetachedTwo()
{
var task = Task.Run(() =>
{
var nestedTask = Task.Run(
() => throw new CustomException("Detached child task faulted."));
// Here the exception will be escalated back to the calling thread.
// We could use try/catch here to prevent that.
nestedTask.Wait();
});
try
{
task.Wait();
}
catch (AggregateException ae)
{
foreach (var e in ae.Flatten().InnerExceptions)
{
if (e is CustomException)
{
Console.WriteLine(e.Message);
}
}
}
}
}
// The example displays the following output:
// Detached child task faulted.
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.
Exceptions that indicate cooperative cancellation
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.
var tokenSource = new CancellationTokenSource();
var token = tokenSource.Token;
var task = Task.Factory.StartNew(() =>
{
CancellationToken ct = token;
while (someCondition)
{
// Do some work...
Thread.SpinWait(50_000);
ct.ThrowIfCancellationRequested();
}
},
token);
// No waiting required.
tokenSource.Dispose();
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)
Using the handle method to filter inner exceptions
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<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.
public static partial class Program
{
public static void HandleMethodThree()
{
var task = Task.Run(
() => throw new CustomException("This exception is expected!"));
try
{
task.Wait();
}
catch (AggregateException ae)
{
// Call the Handle method to handle the custom exception,
// otherwise rethrow the exception.
ae.Handle(ex =>
{
if (ex is CustomException)
{
Console.WriteLine(ex.Message);
}
return ex is CustomException;
});
}
}
}
// The example displays the following output:
// This exception is expected!
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.
public static partial class Program
{
public static void TaskException()
{
// This should throw an UnauthorizedAccessException.
try
{
if (GetAllFiles(@"C:\") is { Length: > 0 } files)
{
foreach (var file in files)
{
Console.WriteLine(file);
}
}
}
catch (AggregateException ae)
{
foreach (var ex in ae.InnerExceptions)
{
Console.WriteLine(
"{0}: {1}", ex.GetType().Name, ex.Message);
}
}
Console.WriteLine();
// This should throw an ArgumentException.
try
{
foreach (var s in GetAllFiles(""))
{
Console.WriteLine(s);
}
}
catch (AggregateException ae)
{
foreach (var ex in ae.InnerExceptions)
Console.WriteLine(
"{0}: {1}", ex.GetType().Name, ex.Message);
}
}
static string[] GetAllFiles(string path)
{
var task1 =
Task.Run(() => Directory.GetFiles(
path, "*.txt",
SearchOption.AllDirectories));
try
{
return task1.Result;
}
catch (AggregateException ae)
{
ae.Handle(x =>
{
// Handle an UnauthorizedAccessException
if (x is UnauthorizedAccessException)
{
Console.WriteLine(
"You do not have permission to access all folders in this path.");
Console.WriteLine(
"See your network administrator or try another path.");
}
return x is UnauthorizedAccessException;
});
return Array.Empty<string>();
}
}
}
// 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.
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.
Observing exceptions by using the Task.Exception property
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.
public static partial class Program
{
public static void ExceptionPropagationTwo()
{
_ = Task.Run(
() => throw new CustomException("task1 faulted."))
.ContinueWith(_ =>
{
if (_.Exception?.InnerException is { } inner)
{
Console.WriteLine("{0}: {1}",
inner.GetType().Name,
inner.Message);
}
},
TaskContinuationOptions.OnlyOnFaulted);
Thread.Sleep(500);
}
}
// The example displays output like the following:
// CustomException: task1 faulted.
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 meaningful application, the continuation delegate could log detailed information about the exception and possibly spawn new tasks to recover from the exception. If a task faults, the following expressions throw the exception:
await tasktask.Wait()task.Resulttask.GetAwaiter().GetResult()
Use a try-catch statement to handle and observe thrown exceptions. Alternatively, observe the exception by accessing the Task.Exception property.
Important
The AggregateException cannot be explicitly caught when using the following expressions:
await tasktask.GetAwaiter().GetResult()
UnobservedTaskException event
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.
