I have a problem with handling the AggregateException if my WinForms application starts a task to keep responsive while the task is performing.
The simplified case is as follows. Suppose my Form has a fairly slow method, for instance:
private double SlowDivision(double a, double b)
{
System.Threading.Thread.Sleep(TimeSpan.FromSeconds(5));
if (b==0) throw new ArgumentException("b");
return a / b;
}
After pressing a button I want my form to show the result of SlowDivision(3,4). The following code would hang the user interface for some time:
private void button1_Click(object sender, EventArgs e)
{
this.label1.Text = this.SlowDivision(3, 4).ToString();
}
Hence I'd like to start a task that will do the processing. When this task finishes, it should continue with an action that will display the result. To prevent an InvalidOperationException I need to be sure that label1 is accessed from the thread that it was created on, hence a Control.Invoke:
private void button1_Click(object sender, EventArgs e)
{
Task.Factory.StartNew ( () =>
{
return this.SlowDivision(3, 4);
})
.ContinueWith( (t) =>
{
this.Invoke( new MethodInvoker(() =>
{
this.label1.Text = t.Result.ToString();
}));
});
}
So far, so good, but how to handle exceptions, for instance If I want to calculate SlowDivision(3, 0)?
Normally if a task throws an unhandled exception, it is forwarded to a waiting thread via an AggregateException. Numerous examples show the following code:
var myTask = Task.Factory.StartNew ( () => ...);
try
{
myTask.Wait();
}
catch (AggregateException exc)
{
// handle exception
}
The problem is: I can't wait for my Task to execute, because I want my UI to remain responsive.
Create a task continuation on faulted that would read Task.Exception and handle accordingly doesn't work:
private void button1_Click(object sender, EventArgs e)
{
var slowDivTask = Task.Factory.StartNew(() =>
{
return this.SlowDivision(3, 0);
});
slowDivTask.ContinueWith((t) =>
{
this.Invoke(new MethodInvoker(() =>
{
this.label1.Text = t.Result.ToString();
}));
}, TaskContinuationOptions.NotOnFaulted);
slowDivTask.ContinueWith((t) =>
{
AggregateException ae = t.Exception;
ae.Handle(exc =>
{
// handle the exception
return true;
});
}, TaskContinuationOptions.OnlyOnFaulted);
}
A try / catch in the function also doesn't help (as could be expected).
So how do I react properly on AggregateExceptions thrown by the task without waiting for it.