I need the same thing, I can find the element ID of an element in a linked file, but I want to highlight it thus adding this element to the selection set.
Simply adding this elementID to the selectionset and giving this to the uidoc doesn't give an error but also doesn't highlight the element in the linked file.
uidoc.Selection.SetElementIds(_elemIdSet);
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Ning_Zhou, did you manage to highlight an element in a linked file? Or can someone tell me how to achieve this ?