Windows phone 8.1 从代码隐藏添加到内容时，WebView不可见

我有下面的代码，我想做的是用一个新的网格替换用户控件的内容，然后这个网格将承载一个webviewer。如果我测试以下代码，webviewer将不可见

        Content = ContentGrid;
        ContentGrid.VerticalAlignment = VerticalAlignment.Stretch;
        ContentGrid.HorizontalAlignment = HorizontalAlignment.Stretch;
        var rectangle = new Rectangle();
        rectangle.Height = 300;
        rectangle.Width = 300;
        rectangle.Fill = new SolidColorBrush(Colors.Red);

        WebView.Name = "CevaSmecher";
        WebView.VerticalAlignment = VerticalAlignment.Stretch;
        WebView.HorizontalAlignment = HorizontalAlignment.Stretch;
        WebView.Height = 300;
        WebView.Height = 300;
        WebView.Visibility = Visibility.Visible;
        WebView.DefaultBackgroundColor = Colors.Red;

        ContentGrid.Name = "SmecherieRoute";
        ContentGrid.Background = new SolidColorBrush(Colors.BlueViolet);
        ContentGrid.Children.Insert(0, WebView);

然后我用这一行替换最后一行，矩形可见

ContentGrid.Children.Insert(0, rectangle);

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我也有同样的问题。我的问题是，在UserControl中，CacheMode被设置为BitmapCache

CacheMode = new BitmapCache();

在我删除这一行之后，出现了WebView