Exception accessing XAML controls inside a task

Question

If I try to access XAML controls inside a task (and task::then) my Metro XAML app always stops with an exception.
The same code works without any problems outside the task. I didn't find any answer - what did I miss?

VS11 Debugger reports: Concurrency::unobserved_task_exception

Exception: The application called an interface that was marshalled for a different thread.

Many thanks for your help!

void MyClass::MyMemberFunction()
{
    xamlStoryboard->Stop(); // ok
    xamlImage->Source = ref new BitmapImage(); // ok

    task<void> atask([this] ()
    {
        xamlStoryboard->Stop(); // exception!
        xamlImage->Source = ref new BitmapImage(); //exception!
    });

    atask.then([this] ()
    {
        xamlStoryboard->Stop(); // exception!
        xamlImage->Source = ref new BitmapImage(); //exception!
    });
}

Answer 1

Hello,

 

How about  task<void> atask([this, xamlStoryboard] (){}?

 

As far as I know, the this pointer to the capture clause to provide access to the methods and data members of the enclosing class.

 

So where do you define the xamlStoryboard

 

Best regards,

Jesse

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Jesse Jiang [MSFT]
MSDN Community Support | Feedback to us

Answer 2

Hello Jesse,

xamlStoryboard is defined in MyClass.xaml

task<void> atask([this, xamlStoryboard] (){}

VS11 Error: Member MyClass::xamlStoryboard is not a variable

But I found:

Creating Asynchronous Operations in C++ for Metro style Apps
http://msdn.microsoft.com/en-us/library/hh750082(v=vs.110).aspx

Chapter "Controlling the Execution Thread" explains, why it isn't possible to directly access XAML controls from inside the task.
Maybe I can use the controls dispatcher?
But I further don't unterstand, why I get exceptions in the task::then continuation code. I thought task::then is the callback code executed after the task finishes and runs in the main thread with access to UI/XAML controls?

best regards,
Tobias

Answer 3

If you pass task_continuation_context::use_current() as a second parameter to the 'then' method then your callback will be scheduled as expected without needing to use the Dispatcher.

For further info see Asynchronous programming in C++ about 2/3 of the way down in 'Managing the thread context'.

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http://babaandthepigman.spaces.live.com/blog/

Answer 4

Thanks BabaAndThePigman for the solution!

Working code is (see postings and links for details):

void MyClass::MyMemberFunction()
{
    xamlStoryboard->Stop(); // ok
    xamlImage->Source = ref new BitmapImage(); // ok

    task<void> atask([this] ()
    {
        xamlStoryboard->Stop(); // exception!
        xamlImage->Source = ref new BitmapImage(); //exception!
    });

    atask.then([this] ()
    {
        xamlStoryboard->Stop(); // ok
        xamlImage->Source = ref new BitmapImage(); // ok
    }, task_continuation_context::use_current()); // that parameter SOLVED the described problems!
}