如何将UTF-8转换为UTF-8n

Encoding 提供一个前导码（即一个字节数组），可以将它作为编码过程中所产生的字节序列的前缀。如果前导码中包含字节顺序标记（在 Unicode 中，码位为 U+FEFF），则它会帮助解码器确定字节顺序和转换格式或 UTF。Unicode 字节顺序标记的序列化结果（十六进制）如下所示：

• UTF-8：EF BB BF

• UTF-16 Big-Endian 字节顺序：FE FF

• UTF-16 Little-Endian 字节顺序：FF FE

• UTF-32 Big-Endian 字节顺序：00 00 FE FF

• UTF-32 Little-Endian 字节顺序：FF FE 00 00

其中，UTF-8的字节顺序标记分别是EF  BB  BF转换成十进制以后是239  298  191 .

当我们通过自己编写的程序读取文件中的数据时候，如果编码不正确，就会导致读取错误。

例如：现在有一个文件，编码类型为UTF-8n(这种编码也属于UTF-8编码，但是文件中不包含字节顺序标记)。如果我们使用C#提供的StreamReader类进行读取，指定的编码为Encoding.Unicode的时候，会错误的读取该数据。但是如果该文件的编码类型为UTF-8则不会读取发生错误，因为我们指定的编码是可以识别UTF－8编码的。如果想正确的读取UTF-8n文件，我们只需要将StreamReader的编码指定为Encoding.UTF－8就可以了。

但是有时候我们需要制作一些数据来进行测试。需要将UTF-8数据转换成UTF-8n或者是将UTF-8n 转换成UTF-8类型。

下边的简单代码演示如何进行转换：

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.IO;
using IWshRuntimeLibrary;

namespace WindowsApplication7
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private string name;
        //UTF-8字符标记
        private const Byte b1 = 239;
        private const Byte b2 = 187;
        private const Byte b3 = 191;
        bool ishavebom = false;
        /**//// <summary>
        /// 清除字符标记
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void button2_Click(object sender, EventArgs e)
        {
            try
            {
                ishavebom = HasBom();

                using (BinaryReader br = new BinaryReader(System.IO.File.Open(this.textBox1.Text, FileMode.Open)))
                {
                    
                    FileInfo fi = new FileInfo(this.textBox1.Text);

                    using (BinaryWriter bw = new BinaryWriter(System.IO.File.Open(fi.DirectoryName + "\\ClearBom_" + fi.Name, FileMode.Create)))
                    {
                        try
                        {
                            if (ishavebom)
                            {
                                br.ReadByte();
                                br.ReadByte();
                                br.ReadByte();
                            }
                            do
                            {
                                bw.Write(br.ReadByte());
                            }
                            while (br != null);
                        }
                        catch (EndOfStreamException)
                        { }
                        bw.Close();
                    }
                    br.Close();
                }
                MessageBox.Show("Over");
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }


        }
        /**//// <summary>
        /// 添加上字符标记
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void button3_Click(object sender, EventArgs e)
        {
            try
            {
                ishavebom = HasBom();
                using (BinaryReader br = new BinaryReader(System.IO.File.Open(this.textBox1.Text, FileMode.Open)))
                {
                    FileInfo fi = new FileInfo(this.textBox1.Text);

                    using (BinaryWriter bw = new BinaryWriter(System.IO.File.Open(fi.DirectoryName + "\\AddBom_" + fi.Name, FileMode.Create)))
                    {
                        try
                        {
                            if (!ishavebom)
                            {
                                bw.Write(b1);
                                bw.Write(b2);
                                bw.Write(b3);
                            }
                            do
                            {
                                bw.Write(br.ReadByte());
                            }
                            while (br != null);
                        }
                        catch (EndOfStreamException)
                        { }
                        bw.Close();
                    }
                    br.Close();
                }
                MessageBox.Show("Over");
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }
        /**//// <summary>
        /// 检查是否存在字符标记
        /// </summary>
        /// <returns></returns>
        private bool HasBom()
        {
            try
            {
                using (BinaryReader br = new BinaryReader(System.IO.File.Open(this.textBox1.Text, FileMode.Open)))
                {
                    FileInfo fi = new FileInfo(this.textBox1.Text);

                    if (br.ReadByte() == b1)
                    {
                        if (br.ReadByte() == b2)
                        {
                            if (br.ReadByte() == b3)
                            {
                                br.Close();
                                return true;
                            }
                        }
                    }
                  
                    br.Close();
                }
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
            return false;
        }
        private void button1_Click(object sender, EventArgs e)
        {
            OpenFileDialog fd = new OpenFileDialog();
            DialogResult dr = fd.ShowDialog(this);
            if (dr == DialogResult.OK)
            {
                this.textBox1.Text = fd.FileName;
            }
        }
    }
}