TaskContinuationOptions.OnlyOnFaulted 被忽略

Question

I am using the Task Parallel Library for .NET 3.5 (a NuGet package ).

When running the following code, the OnlyOnFaulted task is not being run when an exception is thrown from MethodThrowsException() :

var task = Task.Factory.StartNew<SomeType>(() =>
{
    MethodThrowsException();
})
.ContinueWith(t =>
{
    ExceptionMessageTextBox.Text = t.Exception.Message;
},
CancellationToken.None, TaskContinuationOptions.OnlyOnFaulted, TaskScheduler.FromCurrentSynchronizationContext());

I (eventually) get this exception:

System.AggregateException was unhandled
Message: An unhandled exception of type 'System.AggregateException' occurred in System.Threading.dll
Additional information: TaskExceptionHolder_UnhandledException

I cannot see where it is located as it is being thrown within System.Threading.dll (I would need to load the pdb).

As far as I can see, I am observing the exception correctly, as specified by this MSDN article .

Answer 1

So I know this question is pretty old, but I encountered this same problem today, and this question was the only one that I could find related to this issue on StackOverflow. I'm going to add my solution in hopes it helps someone else in the same situation.

The problem here is that the thrown exception is not actually being handled by the continuation. The exception(s) thrown within the task remain unhandled individually, therefore we must actually handle them. Include the code below inside the continuation and it should resolve the issue:

backgroundTask.ContinueWith(t =>
{
    if (t.Exception != null)
    {
        var flattenedExceptions = t.Exception.Flatten();
        flattenedExceptions.Handle((exp) => {
            // process each exception here
            return true;
        });
    }

}, TaskContinuationOptions.OnlyOnFaulted);

More information here at the MSDN documentation .