判断文件编码格式

读取前四个字节判断文件类型

        /// <summary>
        /// 获取文件编码
        /// </summary>
        /// <param name="fs">文件流</param>
        /// <returns></returns>
        public static Encoding GetEncoding(FileStream fs)
        {
            var bom = new byte[4];
            fs.Read(bom, 0, 4);

            // Analyze the BOM
            if (bom[0] == 0x2b && bom[1] == 0x2f && bom[2] == 0x76)
            {
                return Encoding.UTF7;
            }
            else if (bom[0] == 0xef && bom[1] == 0xbb && bom[2] == 0xbf)
            {
                return Encoding.UTF8;
            }
            else if (bom[0] == 0xff && bom[1] == 0xfe)
            {
                return Encoding.Unicode; //UTF-16LE
            }
            else if (bom[0] == 0xfe && bom[1] == 0xff)
            {
                return Encoding.BigEndianUnicode; //UTF-16BE
            }
            else if (bom[0] == 0 && bom[1] == 0 && bom[2] == 0xfe && bom[3] == 0xff)
            {
                return Encoding.UTF32;
            }
            else
            {
                return Encoding.ASCII;
            }
        }