var RVT_Links = new FilteredElementCollector(doc, doc.ActiveView.Id)
.OfCategory(BuiltInCategory.OST_RvtLinks)
.Cast<RevitLinkInstance>();
List<Document> linkDoc_lst = new List<Document>();
foreach (RevitLinkInstance x in RVT_Links)
{
Document linkDoc = x.GetLinkDocument();
linkDoc_lst.Add(linkDoc);
}
ICollection<ElementId> cat_icolls = new Collection<ElementId>();
cat_icolls.Add(new ElementId(BuiltInCategory.OST_Floors));
cat_icolls.Add(new ElementId(BuiltInCategory.OST_Walls));
ElementMulticategoryFilter mcats_filter = new ElementMulticategoryFilter(cat_icolls);
var linkDoc_els = linkDoc_lst.Select(x => new FilteredElementCollector(x).WherePasses(mcats_filter).WhereElementIsNotElementType().ToElements()).SelectMany(x => x);