[C#-Util]-读写文件汉字乱码

     当我们用System.IO.StreamReader读取包含汉字的txt文件时，经常会读出乱码（StreamWriater写文本文件也有类似的问题），原因很简单，就是文件的编码（encoding）和StreamReader/Writer的encoding不对应。
    为了解决这个问题，我写了一个类，来取得一个文本文件的encoding，这样我们就可以创建对应的StreamReader和StreamWriter来读写，保证不会出现乱码现象。其实原理很简单，文本编辑器（比如XP自带的记事本）在生成文本文件时，如果编码格式和系统默认的编码（中文系统下默认为GB2312）不一致时，会在txt文件开头部分添加特定的“编码字节序标识（Encoding Bit Order Madk，简写为BOM）”，类似PE格式的"MZ"文件头。这样它在读取时就可以根据这个BOM来确定该文本文件生成时所使用的Encoding。这个BOM我们用记事本等程序打开默认是看不到的，但是用stream按字节读取时是可以读到的。我的这个TxtFileEncoding类就是根据这个BOM“文件头”来确定txt文件生成时用到的编码的。

using System;

using System.IO;

using System.Text;



namespace XXX.XXX.XXX.Common

{

    public class EncodingHelper

    {

        /// <summary>

        /// GetEncoding

        /// </summary>

        /// <param name="fileName"></param>

        /// <returns></returns>

        public static Encoding GetEncoding(string fileName)

        {

            return GetEncoding(fileName, Encoding.Default);

        }



        /// <summary>

        /// GetEncoding

        /// </summary>

        /// <param name="stream"></param>

        /// <returns></returns>

        public static Encoding GetEncoding(FileStream stream)

        {

            return GetEncoding(stream, Encoding.Default);

        }



        /// <summary>

        /// GetEncoding

        /// </summary>

        /// <param name="fileName"></param>

        /// <param name="defaultEncoding"></param>

        /// <returns></returns>

        public static Encoding GetEncoding(string fileName, Encoding defaultEncoding)

        {

            var fs = new FileStream(fileName, FileMode.Open);

            var targetEncoding = GetEncoding(fs, defaultEncoding);

            fs.Close();

            return targetEncoding;

        }



        /// <summary>

        /// GetEncoding

        /// </summary>

        /// <param name="stream"></param>

        /// <param name="defaultEncoding"></param>

        /// <returns></returns>

        public static Encoding GetEncoding(FileStream stream, Encoding defaultEncoding)

        {

            var 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;

                }



                if (byte1 == 0xFF && byte2 == 0xFE && byte3 != 0xFF)//Unicode

                {

                    targetEncoding = Encoding.Unicode;

                }



                if (byte1 == 0xEF && byte2 == 0xBB && byte3 == 0xBF)//UTF8

                {

                    targetEncoding = Encoding.UTF8;

                }



                //恢复Seek位置       

                stream.Seek(origPos, SeekOrigin.Begin);

            }



            return targetEncoding;

        }



        /// <summary>

        /// GetEncoding

        /// </summary>

        /// <param name="bytes"></param>

        /// <param name="defaultEncoding"></param>

        /// <returns></returns>

        private static Encoding GetEncoding(byte[] bytes, Encoding defaultEncoding)

        {

            var targetEncoding = defaultEncoding;



            byte byte1 = 0;

            byte byte2 = 0;

            byte byte3 = 0;



            if (bytes.Length >= 1)

            {

                byte1 = bytes[0];

            }



            if (bytes.Length >= 2)

            {

                byte2 = bytes[1];

            }



            if (bytes.Length >= 3)

            {

                byte3 = bytes[2];

            }



            if (byte1 == 0xFE && byte2 == 0xFF)

            {

                targetEncoding = Encoding.BigEndianUnicode;

            }



            if (byte1 == 0xFF && byte2 == 0xFE && byte3 != 0xFF)

            {

                targetEncoding = Encoding.Unicode;

            }



            if (byte1 == 0xEF && byte2 == 0xBB && byte3 == 0xBF)

            {

                targetEncoding = Encoding.UTF8;

            }



            return targetEncoding;

        }

    }

}