C# Dictionary通过value获取对应的key值

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
}