ReadLines

        foreach (var item in ReadLines("E:\\bigFile.txt"))
        {
            Console.Write(item);
        }

        /// <summary>
        /// 异步读取每行内容
        /// </summary>
        /// <param name="fileName">指定文件的绝对路径</param>
        /// <returns></returns>
        public static IEnumerable<string> ReadLines(string fileName)
        {
            return ReadLines(delegate
            {
                return File.OpenText(fileName);
            });
        }

        private static IEnumerable<string> ReadLines(Func<TextReader> provider)
        {
            using (TextReader reader = provider())
            {
                string line;
                while ((line = reader.ReadLine()) != null)
                {
                    yield return line;
                }
            }
        }

来源：https://www.cnblogs.com/wesson2019-blog/p/11938313.html