Dictionary通过Value找到它的key

 private void GetDicKeyByValue()
         {
             Dictionary<string, string> dic = new Dictionary<string, string>();
             dic.Add("", "");
             dic.Add("", "");
             dic.Add("", "");
             //foreach KeyValuePair traversing
             foreach (KeyValuePair<string, string> kvp in dic)
             {
                 if (kvp.Value.Equals(""))
                 {
                     //...... kvp.Key;
                 }
             }

             //foreach dic.Keys
             foreach (string key in dic.Keys)
             {
                 if (dic[key].Equals(""))
                 {
                     //...... key
                 }
             }

             //Linq
             var keys = dic.Where(q => q.Value == "").Select(q => q.Key);  //get all keys

             List<string> keyList = (from q in dic
                                     where q.Value == ""
                                     select q.Key).ToList<string>(); //get all keys

             var firstKey = dic.FirstOrDefault(q => q.Value == "").Key;  //get first key
         }