[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;
        }
    }
}