Windows Phone 8 maps

我有下一个XAML：

<maps:Map x:Name="MyMap"  HorizontalAlignment="Left" Margin="10,10,0,0" VerticalAlignment="Top" Height="410" Width="436" CartographicMode="Hybrid">
            <toolkit:MapExtensions.Children>
                <toolkit:Pushpin Visibility="Collapsed" x:Name="pushpin" Content="My Position"/>
            </toolkit:MapExtensions.Children>
        </maps:Map>

这是我的C#代码：

string[] tmp = result.ToString().Split(' ');
        MyMap.Center = new GeoCoordinate(double.Parse(tmp[0]), double.Parse(tmp[1]));
        MapLayer layer0 = new MapLayer();
        Pushpin pushpin0 = new Pushpin();
        pushpin0.Visibility = Visibility.Visible;
        pushpin0.GeoCoordinate = new GeoCoordinate(double.Parse(tmp[0]), double.Parse(tmp[1]));
        MapOverlay overlay0 = new MapOverlay();
        overlay0.Content = pushpin0;
        overlay0.GeoCoordinate = new GeoCoordinate(double.Parse(tmp[0]), double.Parse(tmp[1]));
        layer0.Add(overlay0);

我的图钉没有出现在地图上，我在这里缺了什么？

MyMap.Layers.Add(layer0);