I don't see any obvious way to do that in ObjectARX.
If the Editor's SelectionAdded event fires before the
first selection is made by the user, that would be the
only way it could be done.
In any case, if you can't get that to work, then I would
just use the ActiveX API to get the selection, and then
use the Editor's SelectPrevious() method to get the
resulting selection as a managed SelectionSEt.
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Using
SelectionSet sSet = .... // create a new selection set
sSet.AddItems(oldEntities); // where oldEntities is AcadEntity[] with the
entities to be pre-selected.
sSet.SelectOnScreen(...); // get user input