Room tmpRoom = document.GetElement(eid) as Room;
Parameter areaParam = tmpRoom.get_Parameter(new Guid("11111111-0000-0000-0000-000000000001"));
ParameterValueProvider pvp = new ParameterValueProvider(areaParam.AsElementId());
// evaluator
FilterStringRuleEvaluator fnrv = new FilterStringEquals();
// rule value
string ruleValue = areaParam.AsString();
// rule
FilterRule fRule = new FilterStringRule(pvp, fnrv, ruleValue, true);
ElementParameterFilter filter = new ElementParameterFilter(fRule);
FilteredElementCollector collector = new FilteredElementCollector(document);
IList<Element> rooms = collector.WherePasses(filter).ToElements();