Question

I have an asynchronously running Task that fires an event when it’s completed like this:

task.ContinueWith(() => {
    if (MyEvent != null)
        MyEvent(this, EventArgs.Empty);
}

The event handler then should create an instance of a WPF control. But when I try to do so, it causes an exception: The calling thread must be STA, because many UI components require this. Exception occurs in the class constructor, when calling method InitializeComponent().

As far as I know, usually accessing WPF controls from separate threads is handled using the Dispatcher.Invoke, and it always worked for me, so I tried it:

Dispatcher.Invoke(new Action(() =>
{
    InitializeComponent();
}));

But in that case exception keeps occurring. How do I create an instance of a WPF control from a separate thread?

Or maybe it will be a better approach to marshal the completion event to the main UI thread. If yes, how can I do that?

Answer 1

You have to use a Dispatcher instance, which was associated with the UI thread. If you are writing something like this:

Dispatcher.Invoke(new Action(() =>
{
    InitializeComponent();
}));

In the task body, you’re using dispatcher of the calling thread, which can be a background thread from a pool.

Anyway, with tasks you shouldn’t use Dispatcher directly. Use an appropriate task scheduler:

var ui = TaskScheduler.FromCurrentSynchronizationContext();
Task.Factory.ContinueWhenAll(tasks.ToArray(),
    result =>
    {
        // Put you UI calls here

    }, CancellationToken.None, TaskContinuationOptions.None, ui);

where tasks is a sequence of tasks being executed with the default scheduler.