Many thanks, Dominique. But I thought geometry is reference type, why it is not and don't need to be deep cloned?
For example the code below, why after the last line is been executed, the gra.Geometry is not null:
MapPoint mapPoint = new MapPoint(2.4, 42.6);
Graphic gra = new Graphic();
gra.Geometry = mapPoint;
mapPoint = null;