The application called an interface that was marshalled for a different thread. (Exception from HRESULT: 0x8001010E (RPC_E_WRONG_THREAD))

Question

In my windows phone 8.1 application I have a singleton service DataService which should once in a while be downloading some data. Meanwhile on UI I should be displaying the amount of data received. DataService.StartGettingData() gets called when user logs into the application:

void StartGettingData() { if (getDataTaskCancellationTokenSource != null) getDataTaskCancellationTokenSource.Cancel(); getDataTaskCancellationTokenSource = new CancellationTokenSource(); var token = getDataTaskCancellationTokenSource.Token; Task.Factory.StartNew(async () => await ExecuteCycleAsync(token), token); } async Task ExecuteCycleAsync(CancellationToken cancellationToken) { while (true) { cancellationToken.ThrowIfCancellationRequested(); await LoadDataAsync(cancellationToken); cancellationToken.ThrowIfCancellationRequested(); await Task.Delay(timeTillNextDownload, cancellationToken); } }

This task will be cancelled when user logs out with the help of

if (getDataTaskCancellationTokenSource != null) getDataTaskCancellationTokenSource.Cancel();

The property containing the result of download looks like this:

List<DataType> Data = new List<DataType>(); public IEnumerable<DataType> Data { get { return Data; } set { Data = value.ToList(); OnDataUpdated(); } } void OnDataUpdated() { var handler = DataUpdated; if (handler != null) handler(this, EventArgs.Empty); }

This part seemed to be working until I had to display the amount of data on the screen. My MainViewModel gets instance of DataService injected with Ninject.

readonly IDataService DataService; public MainViewModel(IDataService dataService) { DataService = dataService; DataService.DataUpdated += DataService_DataUpdated; UpdateDataCount(); } void DataService_DataUpdated(object sender, EventArgs e) { UpdateDataCount(); } void UpdateDataCount() { DataCount = DataService.Data.Count(); }

In xaml I've got TextBlock binded to DataCount property of MainViewModel

int DataCount; public int DataCount { get { return DataCount; } set { DataCount = value; OnPropertyChanged(); } } protected void OnPropertyChanged([CallerMemberName] string propertyName = "") { PropertyChangedEventHandler handler = PropertyChanged; if (handler != null) { handler(this, new PropertyChangedEventArgs(propertyName)); } }

And here is where problem appears: OnPropertyChanged fails with "The application called an interface that was marshalled for a different thread. (Exception from HRESULT: 0x8001010E (RPC_E_WRONG_THREAD))" which get's caught in DataService.LoadDataAsync(). I understand the runtime is trying to tell me I am accessing UI element from non ui thread. But am I? I thought OnPropertyChanged is the magic place which disconnects UI from the rest of background tasks. Of course, the problem can be solved implementing OnPropertyChanged this way:

public CoreDispatcher Dispatcher { get; set; } protected async void OnPropertyChanged([CallerMemberName] string propertyName = "") { PropertyChangedEventHandler handler = PropertyChanged; if (handler != null) { await Dispatcher.RunAsync(Windows.UI.Core.CoreDispatcherPriority.Normal, () => { handler(this, new PropertyChangedEventArgs(propertyName)); }); } }

But should it really be implemented this way? Or am I missing something in DataService.ExecuteCycleAsync()?

where do you create a new instance of your view model and how is it getting hooked up to your xaml? — Tejas Sharma
– Tejas Sharma, Commented Nov 24, 2014 at 16:48
Whether you are running in a separate thread depends on where OnDataUpdated is called from. I suspect it is in the LoadDataAsync method which you have ommited and if so, then yes, it is in a separate thread. There is no magic to make the UI thread proof so your approach is correct. — Kell
– Kell, Commented Nov 24, 2014 at 16:52
@Kell, OnDataUpdated is called inside IEnumerable<DataType> Data, so I suppose it is being run on background thread. — foxanna
– foxanna, Commented Nov 24, 2014 at 20:38
@TejasSharma My xaml Page's DataContext is bound to ViewModel property: DataContext="{Binding ViewModel, RelativeSource={RelativeSource Self}}" and it's value is initialized through Ninject as well: ViewModel = NinjectKernel.Get(TypeOfViewModel) — foxanna
– foxanna, Commented Nov 24, 2014 at 20:49

Community · Accepted Answer · 2017-05-23 12:13:53Z

Not trying to dig any deeper, I believe your problem is this:

Task.Factory.StartNew(async () => await ExecuteCycleAsync(token), token);

Change it to simply this, see if it works as expected:

ExecuteCycleAsync(token);

Otherwise, the code inside ExecuteCycleAsync starts execution on a thread without synchronization context, which can lead to all different kinds of problems, depending on what's inside LoadDataAsync.

Note that calling ExecuteCycleAsync(token) like this is still a fire-and-forget call which may not be observing any exceptions (more here). You may want to store the Task object it returns, to be able to observe it later.

Collectives™ on Stack Overflow

The application called an interface that was marshalled for a different thread. (Exception from HRESULT: 0x8001010E (RPC_E_WRONG_THREAD))

1 Answer 1

Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Linked

Related