var filter = new ElementClassFilter(typeof(RevitLinkInstance));
var firstInstanceLink
= (RevitLinkInstance) new FilteredElementCollector(doc)
.WherePasses(filter)
.FirstElement();
filter = new ElementClassFilter(typeof(Floor));
var elemsFromRevitLinkInstance
= new FilteredElementCollector(
firstInstanceLink.GetLinkDocument())
.WherePasses(filter)
.ToElements();
var refs = elemsFromRevitLinkInstance.Select( x
=> new Reference(x).CreateLinkReference(firstInstanceLink))
.ToList();
uidoc.Selection.SetReferences(refs);
uidoc.Application.PostCommand(
RevitCommandId.LookupPostableCommandId(
PostableCommand.HideElements));