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

           }

       }

 }