后台任务(可写入位图)将在windows phone中终止

我正在为windows phone 8.1更新我的应用程序，我想创建一个透明的实时互动程序。我正在使用ToolStack PNG库来创建PNG图像，它第一眼就很好！我在后台任务中使用以下代码

private void CreateWideTile()
    {
        int width = 691;
        int height = 336;
        string imagename = "WideBackground";
        WriteableBitmap b = new WriteableBitmap(width, height);
        var canvas = new Grid();
        canvas.Width = b.PixelWidth;
        canvas.Height = b.PixelHeight;
        var textBlock = new TextBlock();
        textBlock.Text = "example text...";
        textBlock.Margin = new Thickness(10, 55, 0, 0);
        textBlock.Width = 140;
        textBlock.Foreground = new SolidColorBrush(Colors.White);
        textBlock.FontSize = 30;
        canvas.Children.Add(textBlock);
        b.Render(canvas, null);
        b.Invalidate();
        using (var store = IsolatedStorageFile.GetUserStoreForApplication())
        {
            if (store.FileExists("/Shared/ShellContent/" + imagename + ".png"))
                store.DeleteFile("/Shared/ShellContent/" + imagename + ".png");
            var stream = store.OpenFile("/Shared/ShellContent/" + imagename + ".png", FileMode.OpenOrCreate);
            b.WritePNG(stream);
            stream.Close();
        }
    }
    private void CreateMediumTile()
    {
        int width = 336;
        int height = 336;
        string imagename = "MediumBackground";
        WriteableBitmap b = new WriteableBitmap(width, height);
        var canvas = new Grid();
        canvas.Width = b.PixelWidth;
        canvas.Height = b.PixelHeight;
        var textBlock = new TextBlock();
        textBlock.Text = "example text...";
        textBlock.Margin = new Thickness(10, 55, 0, 0);
        textBlock.Width = 140;
        textBlock.Foreground = new SolidColorBrush(Colors.White);
        textBlock.FontSize = 30;
        canvas.Children.Add(textBlock);
        b.Render(canvas, null);
        b.Invalidate(); 
        using (var store = IsolatedStorageFile.GetUserStoreForApplication())
        {
            if (store.FileExists("/Shared/ShellContent/" + imagename + ".png"))
                store.DeleteFile("/Shared/ShellContent/" + imagename + ".png");
            var stream = store.OpenFile("/Shared/ShellContent/" + imagename + ".png", FileMode.OpenOrCreate);
            b.WritePNG(stream);
            stream.Close();
        }
    }
    private void CreateSmallTile()
    {
        int width = 159;
        int height = 159;
        string imagename = "SmallBackground";
        WriteableBitmap b = new WriteableBitmap(width, height);
        var canvas = new Grid();
        canvas.Width = b.PixelWidth;
        canvas.Height = b.PixelHeight;
        var textBlock = new TextBlock();
        textBlock.Text = "example text...";
        textBlock.Margin = new Thickness(10, 55, 0, 0);
        textBlock.Width = 140;
        textBlock.Foreground = new SolidColorBrush(Colors.White);
        textBlock.FontSize = 30;
        canvas.Children.Add(textBlock);
        b.Render(canvas, null);
        b.Invalidate();
        using (var store = IsolatedStorageFile.GetUserStoreForApplication())
        {
            if (store.FileExists("/Shared/ShellContent/" + imagename + ".png"))
                store.DeleteFile("/Shared/ShellContent/" + imagename + ".png");
            var stream = store.OpenFile("/Shared/ShellContent/" + imagename + ".png", FileMode.OpenOrCreate);
            b.WritePNG(stream);
            stream.Close();
        }
    }

正如我提到的，这段代码运行良好，但运行5次后，后台任务将终止，不再运行。。。我真的很困惑。我搜索了所有的互联网，我只是猜测问题是因为内存泄漏。但我不知道如何解决这个令人困惑的问题。我还使用了GC.Collect（）；但这一点帮助都没有。请给一些建议