First or Default 값 가져오기

            List<double> doubleList = new List<double>();
            double a = doubleList.FirstOrDefault();
            //a => 0
            List<string> stringList = new List<string>();
            string b = stringList.FirstOrDefault();
            //b => null
            List<bool> boolList = new List<bool>();
            bool c = boolList.FirstOrDefault();
            //c => false
            List<Regex> regexList = new List<Regex>();
            Regex d = regexList.FirstOrDefault();
            //d => null
            List<int> intList = new List<int>();
            int e = intList.FirstOrDefault();
            // e => 0