//封装的方法//读取文件的值，放入集合中        public static IEnumerable<string> ReadLines(string fileName) {            using (TextReader reader=File.OpenText(fileName)) {                string line;                while ((line=reader.ReadLine())!=null) {                    yield return line;                }            } }//调用 class Program {        static void Main(string[] args) {            foreach (var item in ReadLines("~/map/123.txt")) {                Console.WriteLine(item);            }            Console.ReadKey(); }//x^n public static IEnumerable<int> Power(int number, int exp) {            int result = 1;            for (int i = 0; i < exp; i++) {                result = result * number;                yield return result;            }        }//调用，结果 2,4,8,16,32 class Program {        static void Main(string[] args) {             foreach (var item in Power(2, 5)) {                Console.WriteLine(item);            }            Console.ReadKey(); }