C#中dictionary如何根据索引值获取Key值

var key = dictionary.ElementAt("索引值").Key;
var value= dic.ElementAt("索引值").Value;

Dictionary<string, string> dic = new Dictionary<string, string>();
dic.Add("06:00", "07:30");
dic.Add("08:00", "11:30");
dic.Add("12:00", "14:00");
dic.Add("14:30", "17:00");
dic.Add("16:00", "21:00");
dic.Add("17:00", "21:00");
dic.Add("19:00", "21:00");

/// <summary>
/// 获取开始和结束时间
/// </summary>
private void GetTime(out DateTime dtStart, out DateTime dtEnd)
{
    var strKey= dic.ElementAt(ran.Next(dic.Count())).Key;
    var strValue = dic[strKey];
    dtStart = DateTime.Parse(strKey);
    dtEnd = DateTime.Parse(strValue);
}

Dictionary<string, int> dict = new Dictionary<string, int>()
{
    {"apple", 1},
    {"banana", 2},
    {"orange", 3},
};
string key = dict.FirstOrDefault(x => x.Value == 2).Key;
Console.WriteLine(key);