TPL Exception handling

Question

http://msdn.microsoft.com/en-us/library/dd997415.aspx

Per the article referenced above I am trying to handle exceptions in a continuatin task. The example I am citing in the above article is this:

var task1 = Task.Factory.StartNew(() =>
{
    throw new MyCustomException("Task1 faulted.");
})
.ContinueWith((t) =>
    {
        Console.WriteLine("I have observed a {0}",
            t.Exception.InnerException.GetType().Name);
    },
    TaskContinuationOptions.OnlyOnFaulted);

My code is:

Task<string> task = Task<string>.Factory.StartNew(() => process.StartTask(this));
task.ContinueWith(CloseDialog, TaskContinuationOptions.OnlyOnFaulted);

In StartTask, I throw an error just like the example. My expectation is that CloseDialog will execute and I can examine task.Exception within that method as shown by the example. However when I throw the exception I the code simply stops with an unhandled exception. Should I be using a try/catch block? If so, where? By the way, I want my continuation task (CloseDialog) to ALWAYS run. I'm just using .OnlyOnFaulted because that is what is shown in the example.

Answer 1

A continuation can find out if an exception was thrown by the antecedent Task by the antecedent task's exception property. The following prints the results of a NullReferenceException to the console

Task task1 = Task.Factory.StartNew (() => { throw null; });
Task task2 = task1.ContinueWith (ant => Console.Write(ant.Exception());

If task1 throws an exception and this exception is not captured/queried by the continuation it is considered unhandled and the application dies. With continuations it is enough to establish the result of the task via the Status keyword

asyncTask.ContinueWith(task =>
{
    // Check task status.
    switch (task.Status)
    {
        // Handle any exceptions to prevent UnobservedTaskException.             
        case TaskStatus.RanToCompletion:
            if (asyncTask.Result)
            {
                // Do stuff...
            }
            break;
        case TaskStatus.Faulted:
            if (task.Exception != null)
                mainForm.progressRightLabelText = task.Exception.InnerException.Message;
            else
                mainForm.progressRightLabelText = "Operation failed!";
        default:
            break;
    }
}

If you don't use continuations you either have to wait on the task in a try/catch block or query a task's Result in a try/catch block

int x = 0;
Task<int> task = Task.Factory.StartNew (() => 7 / x);
try
{
    task.Wait();
    // OR.
    int result = task.Result;
}
catch (AggregateException aggEx)
{
    Console.WriteLine(aggEx.InnerException.Message);
}

Hope this help even if it is a bit late and you know all there is by now! :]