[quote user="EvanH"]I would like to use a fence ...[/quote]
I found this note about the DgnElementSetTool class in MicroStationAPI help...
The DgnElementSetTool class can be used to implement a modify command.
Simplifies dealing with the common element sources. Basic tool operation is to populate an ElementAgenda from the active fence, selection sets, or user locate. The tool's _OnElementModify method will then be called for each entry in the ElementAgenda
In other words, DgnElementSetTool lets us deal with a list of candidate elements in an ElementAgenda, irrespective of the source of its contents.
It also provides enum ClipResult, which seems useful in your case...
| enum ClipResult |
Helps determine the action ModifyAgenda will take on the agenda elements after calling DoFenceClip.