LocalGeometryService.GetServiceAsync(localGeometryService => { // Create a new GeometryService task instance. geometryTask = new GeometryService(); // Set the Url property to the Url of the LocalGeometryService. geometryTask.Url = localGeometryService.UrlGeometryService; // Register event handlers for the BufferCompleted and Failed events. geometryTask.UnionCompleted += GeometryService_UnionCompleted; geometryTask.Failed += GeometryService_Failed; }); //after query and got all the features void QueryTask_ExecuteCompleted(object sender, ESRI.ArcGIS.Client.Tasks.QueryEventArgs args) { FeatureSet featureSet = args.FeatureSet; selectedGraphics = featureSet.Features as List<Graphic>; //start union geometryTask.UnionAsync(selectedGraphics); }