ArcGIS API for Silverlight 中根据坐标点在地图上打标记

public void AddMarkerGraphics()
{
ESRI.ArcGIS.Client.Projection.WebMercator mercator = new ESRI.ArcGIS.Client.Projection.WebMercator();
GraphicsLayer graphicsLayer = myMap.Layers["MyGraphicsLayer"] as GraphicsLayer;
Graphic graphic = new Graphic()
{
Geometry = mercator.FromGeographic(new MapPoint(115.257113, 33.0696150000001)),
Symbol = LayoutRoot.Resources["DefaultMarkerSymbol"] as Symbol
};
graphicsLayer.Graphics.Add(graphic);
}