A Multipart is based upon the parent Geometry class. The Geometry class is immutable which means that you can not change its shape once it is created. If you need to modify a Multipart once it has been created, use the MultipartBuilder class instead. The MultipartBuilder.ToGeometry method will provide you with the base Geometry object.
System.Object
ArcGIS.Core.Geometry.Geometry
ArcGIS.Core.Geometry.Multipart
ArcGIS.Core.Geometry.Polygon
ArcGIS.Core.Geometry.Polyline
Target Platforms: Windows 10, Windows 8.1