var doc = commandData.Application.ActiveUIDocument.Document;
 FilteredElementCollector linkInstances = new FilteredElementCollector(doc);
 linkInstances = linkInstances.WherePasses(new ElementClassFilter(typeof(RevitLinkInstance)));

 if (linkInstances != null && linkInstances.Any())
 {
       foreach (RevitLinkInstance linkIns in linkInstances)
       {
           Autodesk.Revit.DB.Document linkDoc = linkIns.GetLinkDocument();
           if (linkDoc != null)
           {
               //选取你想要的构件         
           }
       }
 }