C# Dictionary已知value获取对应的key

1：循环遍历法，分为遍历key-value键值对和遍历所有key两种形式

2：使用Linq查询法

     private void GetDictKeyByValue()
     {
         Dictionary<int, string> dict = new Dictionary<int, string>();
         dict.Add(, "");
         dict.Add(, "");
         dict.Add(, "");
         dict.Add(, "");

         // foreach KeyValuePair
         List<int> list = new List<int>();
         foreach (KeyValuePair<int, string> kvp in dict)
         {
             if (kvp.Value.Equals(""))
             {
                 list.Add(kvp.Key); // kvp.Key;
             }
         }

         // foreach dic.Keys
         list.Clear();
         foreach (int key in dict.Keys)
         {
             if (dict[key].Equals(""))
             {
                 list.Add(key); // key
             }
         }

         // Linq
         List<int> keyList = dict.Where(q => q.Value == "")
                         .Select(q => q.Key).ToList<int>(); //get all keys

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

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