Linq排序，获取前5条数据
_dic = _dic.OrderByDescending(x => x.Value).ToDictionary(x=>x.Key,x=>x.Value);
var Num = _dic.ToList().Take(5).Select(a => a.Key.ToString()).ToArray();
var Str = string.Join(",", Num); 
_dic = _dic.OrderByDescending(x => x.Value).ToDictionary(x=>x.Key,x=>x.Value);
var Num = _dic.ToList().Take(5).Select(a => a.Key.ToString()).ToArray();
var Str = string.Join(",", Num);