文件读取方法（由深到浅）

　　  /// <summary>
        /// 读取文件
        /// </summary>
        /// <param name="filename">文件路径</param>
        /// <param name="mode">文件操作方式</param>
        /// <returns>文件内容</returns>      
        public string ReadFile(string filename, FileMode mode)
        {
            FileStream fs = new FileStream(filename, mode);
            StreamReader sr = new StreamReader(fs);
            string content = sr.ReadToEnd();
            sr.Close();
            fs.Close();
            return content;
        }

        /// <summary>
        /// 读取文件
        /// </summary>
        /// <param name="filename">文件路径</param>
        /// <returns>文件内容</returns>      
        public string ReadFileSimple(string filename)
        {
            using (System.IO.StreamReader sr = new System.IO.StreamReader(filename))
            {
                return sr.ReadToEnd();
            }
        }

        /// <summary>
        /// 读取文件
        /// </summary>
        /// <param name="filename">文件路径</param>
        /// <returns>文件内容</returns>      
        public string ReadFile(string filename)
        {
            return File.ReadAllText(filename);
        }