TaskScheduler.UnobservedTaskException event handler never being triggered

问题

I'm reading through a book about the C# Task Parallel Library and have the following example but the TaskScheduler.UnobservedTaskException handler is never being triggered. Can anyone give me any clues as to why?

TaskScheduler.UnobservedTaskException += (object sender, UnobservedTaskExceptionEventArgs eventArgs) =>
{
    eventArgs.SetObserved();
    ((AggregateException)eventArgs.Exception).Handle(ex =>
    {
        Console.WriteLine("Exception type: {0}", ex.GetType());
        return true;
    });
};

Task task1 = new Task(() => 
{
    throw new ArgumentNullException();
});

Task task2 = new Task(() => {
    throw new ArgumentOutOfRangeException();
});

task1.Start();
task2.Start();

while (!task1.IsCompleted || !task2.IsCompleted)
{
    Thread.Sleep( 5000 );
}

Console.WriteLine("done");
Console.ReadLine();

回答

Unfortunately, that example will never show you your code. The UnobservedTaskException will only happen if a Task gets collected by the GC with an exception unobserved - as long as you hold a reference to task1 and task2, the GC will never collect, and you'll never see your exception handler.

In order to see the behavior of the UnobservedTaskException in action, I'd try the following (contrived example):

public static void Main()
{
    TaskScheduler.UnobservedTaskException += (object sender, UnobservedTaskExceptionEventArgs eventArgs) =>
                {
                    eventArgs.SetObserved();
                    ((AggregateException)eventArgs.Exception).Handle(ex =>
                    {
                        Console.WriteLine("Exception type: {0}", ex.GetType());
                        return true;
                    });
                };

    Task.Factory.StartNew(() =>
    {
        throw new ArgumentNullException();
    });

    Task.Factory.StartNew(() =>
    {
        throw new ArgumentOutOfRangeException();
    });


    Thread.Sleep(100);
    GC.Collect();
    GC.WaitForPendingFinalizers();

    Console.WriteLine("Done");
    Console.ReadKey();
}

This will show you your messages. The first Thread.Sleep(100) call provides enough time for the tasks to throw. The collect and wait forces a GC collection, which will fire your event handler 2x.

In .NET 4.5, you need extra voodoo for this to happen. First, you need to enable ThrowUnobservedTaskException in the app.config, and second, you need to build the code in the Release configuration (a point that's only documented in the Example section of the linked MSDN article). I blame this "let's suppress exceptions silently" mentality on JavaScript in browsers :)

– Roman Starkov

Apr 30, 2013 at 14:05

ThrowUnobservedTaskException is not required for this to work. It is only necessary if you want to revert to the 4.0 behavior of a Task exception terminating the process. In 4.5 the default changed to not terminate the process. It is not required for the unobserved exception to be caught by the event handler. More info in the remarks here

– BitMask777

Sep 21, 2015 at 22:48