Question

Client.GetAsync appears to fail for me when running in a background task in my Windows 8 Metro app.

I tried using both a TimeTrigger and a MaintenanceTrigger. It appears that there is no exception. When debugging it, it just exits at that line (if I keep pressing step over), even though there is way more to execute as well as a try-catch around it.

This leads me to believe that Windows is cancelling my task. So I listened to the cancelled event with no luck.

I have declared Internet access as a capability, and the task has been properly registered. Furthermore, my app has full lock screen access.

What am I doing wrong?

Here’s the code I use to register the background task currently.

if (await ObtainLockScreenAccess())
{
    const string name = "Task";

    var updaterTasks = BackgroundTaskRegistration.AllTasks.Where(t => t.Value.Name == name);
    if (!updaterTasks.Any())
    {

        var builder = new BackgroundTaskBuilder();
        builder.Name = name;
        builder.TaskEntryPoint = "Stackstabilizer.BackgroundTasks.UpdateBackgroundTask";

        var trigger = new MaintenanceTrigger(15, false);
        var condition = new SystemCondition(SystemConditionType.InternetAvailable);

        builder.AddCondition(condition);
        builder.SetTrigger(trigger);

        var task = builder.Register();

        Debug.WriteLine("Task has been registered");

    }
}

Edit Here’s the background task signature:

namespace Stackstabilizer.BackgroundTasks
{
    // You must use a sealed class, and make sure the output is a WINMD. 
    public sealed class UpdateBackgroundTask : IBackgroundTask
    {
        public async void Run(IBackgroundTaskInstance taskInstance)
        {

            var deferral = taskInstance.GetDeferral();

            try { ... } finally { deferral.Complete(); }

        }
     }

}

And here’s some details on my declarations.

Answer 1

I had a similar problem to you before, and the way I solved it was to have another deferral in the async task. I think the idea is that, whenever you have an async task, you need a deferral. (I’m not sure if it’s best practice, but hey, it works)

Code:

    public void Run(IBackgroundTaskInstance taskInstance)
    {
        updateAllTiles(taskInstance);
    }


    public async void updateAllTiles(IBackgroundTaskInstance taskInstance)
    {
        // Need to use a deferral to run async tasks in the background task
        BackgroundTaskDeferral deferral = taskInstance.GetDeferral();

        dosomething(taskInstance);

        deferral.Complete();
    }

    private async void dosomething(IBackgroundTaskInstance taskInstance)
    {
        BackgroundTaskDeferral deferral = taskInstance.GetDeferral();

        HttpClient client = new HttpClient();
        HttpResponseMessage resp = await client.GetAsync(new Uri(url));

        updateTile(resp);

        deferral.Complete();
    }