主要的坐标系有2种：

WKID=102100

WKID=4326

而且对于点来说，一个Geometry对象其Width和Height是0，这点需要特别注意。

            //坐标点聚焦
            ESRI.ArcGIS.Client.Projection.WebMercator mercator = new ESRI.ArcGIS.Client.Projection.WebMercator();
            ESRI.ArcGIS.Client.Geometry.Envelope selectedFeatureExtent = mercator.ToGeographic(graphic.Geometry).Extent; //最后一个点的位置
            double expandPercentage = 30;

            double widthExpand = (selectedFeatureExtent.Width + 5) * (expandPercentage / 100);
            double heightExpand = (selectedFeatureExtent.Height + 5) * (expandPercentage / 100);
            ESRI.ArcGIS.Client.Geometry.Envelope displayExtent = new ESRI.ArcGIS.Client.Geometry.Envelope(
            selectedFeatureExtent.XMin - (widthExpand / 2),
            selectedFeatureExtent.YMin - (heightExpand / 2),
            selectedFeatureExtent.XMax + (widthExpand / 2),
            selectedFeatureExtent.YMax + (heightExpand / 2));
            myMap.ZoomTo(displayExtent);

double widthExpand = (selectedFeatureExtent.Width + 5) * (expandPercentage / 100);

注意这里，给每个Width加上5，这样就可以实现了。

设置坐标系的方法：

 ESRI.ArcGIS.Client.Geometry.Envelope initialExtent =
                    new ESRI.ArcGIS.Client.Geometry.Envelope(
                _mercator.FromGeographic(
                    new ESRI.ArcGIS.Client.Geometry.MapPoint(-122.4545596, 37.783443296)) as MapPoint,
                _mercator.FromGeographic(
                    new ESRI.ArcGIS.Client.Geometry.MapPoint(-122.4449924, 37.786447331)) as MapPoint);

            initialExtent.SpatialReference = new SpatialReference(102100);  //WKID=4326

            MyMap.Extent = initialExtent;