I've searched through previous posts about implementing this method to
customize the child controls being dragged into a custom container control
and I do have some questions.

The CanParent method is only executed if the control dragged into the
container control is from the form and not from toolbox (I think this is a
fact).  I don't know why the behavior is this.

Using Reflector, I saw that the IOleDragClient.IsDropOk(IComponent
component) method of the ParentControlDesigner calls the CanParent and the
CanBeParentTo methods of the involved controls (parent and child).  Who is
calling this method and when was it called?