遍历--委托参数

        #region demo_13
        public delegate TResult Func<TResult>();
        static void demo_13()
        {
            Console.WriteLine("demo_12 ：");
            foreach (string line in ReadLine("text.txt"))
            {
                Console.WriteLine(line);
            }
        }
        static IEnumerable<string> ReadLine(string filename)
        {
            return ReadLine(filename, Encoding.UTF8);
        }
        static IEnumerable<string> ReadLine(string filename, Encoding encoding)
        {
            return ReadLine(delegate { return new StreamReader(filename, encoding); });
        }
        static IEnumerable<string> ReadLine(Func<TextReader> provider)
        {
            using (TextReader reader = provider())
            {
                string line;
                while((line=reader.ReadLine())!=null)
                {
                    yield return line;
                }
            }
        }
        #endregion

来自：深入理解C#第2版