I would implement something like selection box in SolidWorks property manager pages. Either listen for a DDrawingDocEvents_NewSelectionNotifyEventHandler to monitor all the selection or just use the update button on your form to load the currently selected dimension on-demand. Then just call the SelectionMgr::GetSelectedObject6 and you will get the pointer to IDisplayDimension Interface. Now you can work with it: change properties, extract values, names etc.
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