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I am building an RSS Reader and I want to add a periodic task that will check for new feed items. If it finds any new items, it will update the Live Tile of the app accordingly.

The problem that I encounter is that I am using the DownloadStringAsync() method to download the feeds and check if they contain new items. So, sometimes the downloading process may take longer than 20 seconds (the time that a periodic task is given in order to complete its actions).

All I want is to ensure that the NotifyComplete() method will be called before the agent gets terminated by the OS, after 20 seconds of action. For this reason, I want to register a dispatcher timer with an interval of 15 seconds, that will call the NotifyComplete() method in its tick event.

However, I tried to declare and use the dispatcher timer, and an invalid cross-thread access error was raised. My Periodic Task code includes the following:

public class ScheduledAgent : ScheduledTaskAgent
{  
    //Register a DispatcherTimer
    DispatcherTimer masterTimer = new DispatcherTimer();

    private static volatile bool _classInitialized;

    public ScheduledAgent()
    {
        if (!_classInitialized)
        {
            _classInitialized = true;
            // Subscribe to the managed exception handler
            Deployment.Current.Dispatcher.BeginInvoke(delegate
            {
                Application.Current.UnhandledException += ScheduledAgent_UnhandledException;
            });
        }

        //Set Timer properties
        masterTimer.Interval = TimeSpan.FromSeconds(15);
        masterTimer.Tick += masterTimer_Tick;
    }

    protected override void OnInvoke(ScheduledTask task)
    {
        //TODO: Add code to perform your task in background
        masterTimer.Start();

        //Call DownloadStringAsync() and perform other tasks...
        //Call NotifyComplete() after the download is complete.
        //
    }


    private void masterTimer_Tick(object sender, EventArgs e)
    {
        masterTimer.Stop();
        //There is no more time left, we must call NotifyComplete() so as to avoid
        //having the periodic task terminated by the OS
        NotifyComplete();
    }
}

The question is, why this is happening and how I can resolve the issue. Thank you in advance!

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1 回答 1

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当您尝试从创建控件的线程以外的线程更改 UI 属性时，会引发跨线程访问错误。

要传递它，请使用控件中存在的 Invoke 方法

代码应如下所示：

Control.Invoke((MethodInvoker)delegate{
//Do your work here for example NotifyComplete();
});
于 2013-07-19T14:16:20.807 回答