取得一个文本文件流的编码方式

#region 取得一个文本文件流的编码方式
        /// <summary>  
        /// 取得一个文本文件流的编码方式。  
        /// </summary>  
        /// <param name="stream">文本文件流。</param>  
        /// <param name="defaultEncoding">默认编码方式。当该方法无法从文件的头部取得有效的前导符时，将返回该编码方式。</param>  
        /// <returns></returns>  
        public string GetEncoding(FileStream stream, string defaultEncoding)
        {
            string targetEncoding = defaultEncoding;
            if (stream != null && stream.Length >= 2)
            {
                //保存文件流的前4个字节  
                byte byte1 = 0;
                byte byte2 = 0;
                byte byte3 = 0;
                byte byte4 = 0;
                //保存当前Seek位置  
                long origPos = stream.Seek(0, SeekOrigin.Begin);
                stream.Seek(0, SeekOrigin.Begin);

                int nByte = stream.ReadByte();
                byte1 = Convert.ToByte(nByte);
                byte2 = Convert.ToByte(stream.ReadByte());
                if (stream.Length >= 3)
                {
                    byte3 = Convert.ToByte(stream.ReadByte());
                }
                if (stream.Length >= 4)
                {
                    byte4 = Convert.ToByte(stream.ReadByte());
                }
                //根据文件流的前4个字节判断Encoding  
                //Unicode {0xFF, 0xFE};  
                //BE-Unicode {0xFE, 0xFF};  
                //UTF8 = {0xEF, 0xBB, 0xBF};  
                if (byte1 == 0xFE && byte2 == 0xFF)//UnicodeBe  
                {
                    targetEncoding = Encoding.BigEndianUnicode.BodyName;
                }
                if (byte1 == 0xFF && byte2 == 0xFE && byte3 != 0xFF)//Unicode  
                {
                    targetEncoding = Encoding.Unicode.BodyName;
                }
                if (byte1 == 0xEF && byte2 == 0xBB && byte3 == 0xBF)//UTF8  
                {
                    targetEncoding = Encoding.UTF8.BodyName;
                }
                //恢复Seek位置   
                stream.Seek(origPos, SeekOrigin.Begin);
            }
            return targetEncoding;
        }
        #endregion