There currently is no SketchEditor for SceneView but you should be able to use PolylineBuilder and GeoViewTapped events. Something like this.
PolylineBuilder builder = new PolylineBuilder(SpatialReferences.Wgs84);
private void MySceneView_GeoViewTapped(object sender, GeoViewInputEventArgs e)
{
builder.AddPoint(e.Location);
var overlay = MySceneView.GraphicsOverlays.FirstOrDefault();
var graphic = overlay.Graphics.FirstOrDefault();
if (graphic == null)
overlay.Graphics.Add(new Graphic(builder.ToGeometry(), new SimpleLineSymbol(SimpleLineSymbolStyle.Solid, Color.Red, 3)));
else
graphic.Geometry = builder.ToGeometry();
}
In sample code above, we use tapped location but if you have 3D objects you want considered (i.e. buildings in ArcGISSceneLayer), you might want to use this location instead.
var location = await MySceneView.ScreenToLocationAsync(e.Position);
Meanwhile, you can try looking into one of our demo apps originally written against ArcGIS Runtime for .NET 10.x. I updated it slightly to use 100.x in this branch. I hope it will help you get started.
