如何使用LockScreen.SetImageUri设置Windows Phone 8锁定屏幕背景

我在IsolatedStorage中有一个映像，我想通过编程将其设置为设备锁定屏幕背景。我的问题是无法获得LockScreen.SetImageUri所需的正确路径。来自引用http://msdn.microsoft.com/en-us/library/windowsphone/develop/jj206968（v=vs.105）.aspx很明显-appdata:///local/"是本地图像所需的前兆。

var schema = isAppResource ? "ms-appx:///" : "ms-appdata:///Local/";
var uri = new Uri(schema + filePathOfTheImage, UriKind.Absolute);

我在我的应用程序IsolatedStorage中创建了一个名为Pictures的文件夹，其中保存了CameraCaptureTask中的jpg图像。我已经尝试了几种方法通过上述方案访问该文件夹中的图像，但我总是在下一行上收到ArgumentException

Windows.Phone.System.UserProfile.LockScreen.SetImageUri(uri);

但是在调试的时候，我看到uri = "ms-appdata:///Local/Pictures/WP_20130812_001.jpg"，这怎么不正确呢？

我的实现如下

private void recent_SelectionChanged(object sender, SelectionChangedEventArgs e)
{            
    capturedPicture = (sender as LongListSelector).SelectedItem as CapturedPicture;
    if (capturedPicture != null)
    {
        //filename is the name of the image in the IsolatedStorage folder named Pictures
        fileName = capturedPicture.FileName;
    }
}
void setAsLockScreenMenuItem_Click(object sender, EventArgs e)
{
    if (!String.IsNullOrEmpty(fileName)) 
    {
        //PictureRepository.IsolatedStoragePath is a string = "Pictures"                
        //LockHelper("isostore:/" + PictureRepository.IsolatedStoragePath + "/" + fileName, false);  //results in FileNotFoundException
        LockHelper(PictureRepository.IsolatedStoragePath + "/" + fileName, false);  //results in ArgumentException
    }
    else
    {
        MessageBoxResult result = MessageBox.Show("You must select an image to set it as your lock screen.", "Notice", MessageBoxButton.OK);
        if (result == MessageBoxResult.OK)
        {
            return;
        }
    }
}
private async void LockHelper(string filePathOfTheImage, bool isAppResource)
{
try
{
    var isProvider = Windows.Phone.System.UserProfile.LockScreenManager.IsProvidedByCurrentApplication;
    if (!isProvider)
    {
        // If you're not the provider, this call will prompt the user for permission.
        // Calling RequestAccessAsync from a background agent is not allowed.
        var op = await Windows.Phone.System.UserProfile.LockScreenManager.RequestAccessAsync();
        // Only do further work if the access was granted.
        isProvider = op == Windows.Phone.System.UserProfile.LockScreenRequestResult.Granted;
    }
    if (isProvider)
    {
        // At this stage, the app is the active lock screen background provider.
        // The following code example shows the new URI schema.
        // ms-appdata points to the root of the local app data folder.
        // ms-appx points to the Local app install folder, to reference resources bundled in the XAP package.
        var schema = isAppResource ? "ms-appx:///" : "ms-appdata:///Local/";
        var uri = new Uri(schema + filePathOfTheImage, UriKind.Absolute);
        // Set the lock screen background image.
        Windows.Phone.System.UserProfile.LockScreen.SetImageUri(uri);
        // Get the URI of the lock screen background image.
        var currentImage = Windows.Phone.System.UserProfile.LockScreen.GetImageUri();
        System.Diagnostics.Debug.WriteLine("The new lock screen background image is set to {0}", currentImage.ToString());
    }
    else
    {
        MessageBox.Show("You said no, so I can't update your background.");
    }
}
catch (System.Exception ex)
{
    System.Diagnostics.Debug.WriteLine(ex.ToString());
}

}

如何将LockScreen.SetImageUri修改为正确的预期uri？