Document doc = Application.DocumentManager.MdiActiveDocument;
Database db = doc.Database;
Editor ed = doc.Editor;
PromptSelectionOptions pso = new PromptSelectionOptions();
pso.MessageForAdding = "\n选择子实体的边";
pso.SingleOnly = true;
pso.ForceSubSelections = true;
PromptSelectionResult psrEdge = ed.GetSelection(pso);
if (psrEdge.Status != PromptStatus.OK) return;
SubentityId retId = new SubentityId();
SelectedObject so = psrEdge.Value[0];
solId = so.ObjectId;
Solid3d solid = tr.GetObject(so.ObjectId, OpenMode.ForRead) as Solid3d;
SelectedSubObject[] sso = so.GetSubentities();
SubentityType subentityType = sso[0].FullSubentityPath.SubentId.Type;

上述代码为我查询到autocad的示例，但是我在zwcad中却不能使用，通过调试发现倒数第二行sso已经为空，从而导致最后一行报空指针异常，请问如何正确使用SelectedSubObject或者有其他api实现选择Solid3d类的子实体功能？