【C#】通过Value获取Dictionary的Key

        Dictionary<string, int> dic = new Dictionary<string, int>();
        for (int i = 0; i < 10; i++)
        {
            dic.Add("string_" + i, i);
        }
        
        //获取第一个符合条件的Key
        //没有符合条件Key, 返回null
        string firstKey = dic.FirstOrDefault(pair => pair.Value == 20).Key;

        //获取所有符合条件的Key
        //没有符合条件Key, 返回空List
        List<string> keys01 = dic.Where(pair => pair.Value == 20).Select(pair => pair.Key).ToList();

        //获取所有符合条件的Key
        List<string> keys02 = (from pair in dic where pair.Value == 7 select pair.Key).ToList();