Dictionary通过Value找到它的key

private void GetDicKeyByValue()
        {
            Dictionary<string, string> dic = new Dictionary<string, string>();
            dic.Add("1", "1");
            dic.Add("2", "2");
            dic.Add("3", "2");
            //foreach KeyValuePair traversing
            foreach (KeyValuePair<string, string> kvp in dic)
            {
                if (kvp.Value.Equals("2"))
                { 
                    //...... kvp.Key;
                }
            }

            //foreach dic.Keys
            foreach (string key in dic.Keys)
            {
                if (dic[key].Equals("2"))
                { 
                    //...... key
                }
            }

            //Linq
            var keys = dic.Where(q => q.Value == "2").Select(q => q.Key);  //get all keys

            List<string> keyList = (from q in dic
                                    where q.Value == "2"
                                    select q.Key).ToList<string>(); //get all keys

            var firstKey = dic.FirstOrDefault(q => q.Value == "2").Key;  //get first key
        }