从Scheduled Agent保存图像时System.UnauthorizedAccessException

Question

我正在构建一个WP8应用程序,它使用来自Internet的图像来更改锁定屏幕的背景.我按照预定代理和锁屏的教程,但我有一个问题.

当我尝试从计划代理下载新的背景图像时,我得到:

+       $exception  {System.UnauthorizedAccessException: Invalid cross-thread access.
   at MS.Internal.XcpImports.CheckThread()
   at System.Windows.DependencyObject..ctor(UInt32 nativeTypeIndex, IntPtr constructDO)
   at System.Windows.Media.Imaging.BitmapImage..ctor()
   at TileLockAgent.ScheduledAgent.lockScreenClient_OpenReadCompleted(Object sender, OpenReadCompletedEventArgs e)
   at System.Net.WebClient.OnOpenReadCompleted(OpenReadCompletedEventArgs e)
   at System.Net.WebClient.OpenReadOperationCompleted(Object arg)
   at System.Threading.WaitCallback.Invoke(Object state)
   at System.Threading.QueueUserWorkItemCallback.WaitCallback_Context(Object state)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.QueueUserWorkItemCallback.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem()
   at System.Threading.ThreadPoolWorkQueue.Dispatch()
   at System.Threading._ThreadPoolWaitCallback.PerformWaitCallback()}   System.Exception {System.UnauthorizedAccessException}

代码是:

string fileName;

try
{
    var currentImage = LockScreen.GetImageUri();

    if (currentImage.ToString().EndsWith("_1.jpg"))
    {
        fileName = "LockBackground_2.jpg";
    }
    else
    {
        fileName = "LockBackground_1.jpg";
    }
}
catch
{
    // lockscreen not set or prev owned by other app          
    fileName = "LiveLockBackground_1.jpg";
}

using (var isoStore = IsolatedStorageFile.GetUserStoreForApplication())
{
    var bi = new BitmapImage();
    bi.SetSource(e.Result);
    var wb = new WriteableBitmap(bi);
    using (var isoFileStream = isoStore.CreateFile(fileName))
    {
        var width = wb.PixelWidth;
        var height = wb.PixelHeight;
        Extensions.SaveJpeg(wb, isoFileStream, width, height, 0, 100);
    }
}

我真的不知道如何解决这个问题.如果BitmapImage不工作,如何在预定的代理中保存图像？我正在做"跨线程访问"是什么意思？图像仅由计划的代理创建和使用,因此没有人应该访问它们.

Answer 1

问题产生于BitmapImage无法在UI线程之外实例化.您可以通过在Dispatcher Invoke调用中包装调用来解决此问题.

但是,您需要确保正确调用NotifyComplete.因此,您可能需要在Dispatcher调用中放置NotifyComplete.

  Deployment.Current.Dispatcher.BeginInvoke(() =>
    {
        UpdateSyncPictureName(...);
        NotifyComplete();
    });

来源:处理隔离存储时,Schedule Agent上的交叉异常无效