Child child = new Child();
child.parentid=parent;
parent.childlist.add(child);

when i session1.update(parent),they will post a Error:

NHibernate.HibernateException: SQL insert, update or delete failed (expected affected row count: 1, actual affected row count: 0). Possible causes: the row was modified or deleted by another user, or a trigger is reporting misleading row count.

who can help me?thanks.

If you have cascading set up correctly, you should save the child: I believe parent.childlist should have inverse="true" on it as well. That's just off the top of my head, though, could be wrong.

thanks,I can cascade child.Because I change child's id befor post parent.I found "unsaved-value" is a import key.
But I found, if i make child's id use "assigned",I can't cascade child. And it will view the same failure as before if I change child's id before post parent.

who can help me? in "assigned" status, how I can cascade child via post parent

If you can, add a version to your object/table, and use the version/@unsaved-value attribute in your mapping. That, I think, is the most elegant solution for assigned-id objects.