[Closed] Add new Modifier to a Node in C#

There are two Modifiers on the Node ,I want to add a new modifier on bottom of the Node.

IIDerivedObject dobj = g.CreateDerivedObject(node.ObjectRef);
int num = ddobj.NumModifiers;
....
IModifier mod = (IModifier)objMod;
dobj.AddModifier(mod, null, num);

The interesting thing is that the “num” is always zero, why?