放大 缩小

 case "放大":
                    MyDrawObject.IsEnabled = true;
                    _toolMode = "zoomin";
                    break;
                case "缩小":
                    MyDrawObject.IsEnabled = true;
                    _toolMode = "zoomout";
                    break;

 void MyDrawObject_DrawComplete(object sender,DrawEventArgs e)
        {
            if (_toolMode == "zoomin")
            {
                MyMap.ZoomTo(e.Geometry as ESRI.ArcGIS.Client.Geometry.Envelope);
            }
            else if (_toolMode == "zoomout")
            {
                Envelope currentExtent = MyMap.Extent;

                Envelope zoomBoxExtent = e.Geometry as Envelope;
                MapPoint zoomBoxCenter = zoomBoxExtent.GetCenter();

                double whRatioCurrent = currentExtent.Width / currentExtent.Height;
                double whRatioZoomBox = zoomBoxExtent.Width / zoomBoxExtent.Height;

                Envelope newEnv = null;

                if (whRatioZoomBox > whRatioCurrent)
                // use width
                {
                    double mapWidthPixels = MyMap.Width;
                    double multiplier = currentExtent.Width / zoomBoxExtent.Width;
                    double newWidthMapUnits = currentExtent.Width * multiplier;
                    newEnv = new Envelope(new MapPoint(zoomBoxCenter.X - (newWidthMapUnits / 2),zoomBoxCenter.Y),new MapPoint(zoomBoxCenter.X + (newWidthMapUnits / 2),zoomBoxCenter.Y));
                }
                else
                // use height
                {
                    double mapHeightPixels = MyMap.Height;
                    double multiplier = currentExtent.Height / zoomBoxExtent.Height;
                    double newHeightMapUnits = currentExtent.Height * multiplier;
                    newEnv = new Envelope(new MapPoint(zoomBoxCenter.X,zoomBoxCenter.Y - (newHeightMapUnits / 2)),new MapPoint(zoomBoxCenter.X,zoomBoxCenter.Y + (newHeightMapUnits / 2)));
                }

                if (newEnv != null)
                    MyMap.ZoomTo(newEnv);
            }
        }

  MyDrawObject = new Draw(MyMap)
            {
                FillSymbol = LayoutRoot.Resources["DefaultFillSymbol"] as ESRI.ArcGIS.Client.Symbols.FillSymbol,DrawMode = DrawMode.Rectangle
            };
            MyDrawObject.DrawComplete += new EventHandler<DrawEventArgs>(MyDrawObject_DrawComplete);