Encoding encode=Encoding.GetEncoding("GB2312")); 可以使用后附的類，先讀文件的編碼格式 encode = fileEncode.GetFileEncodeType("in_file.txt"); string strStr1 = File.ReadAllText("in.txt", encode);

 StreamWriter sw = new StreamWriter("out.txt", false, Encoding.GetEncoding("ASCII")); sw.Write("12.3"); sw.Close();

using System;using System.Collections.Generic;using System.Linq;using System.Xml;using System.IO;using System.Configuration;using System.Text;namespace WebApplication1{//=1=按文件編碼格式讀寫  public partial class _Default : System.Web.UI.Page  {    string proj_name = "";    protected void Page_Load(object sender, EventArgs e)    {      string xml_in_file="c://part.xml";//輸入片段，其它程序生成的是一個節點      string xml_out_file="c://all.xml";//整體，新節點片段，要追加到其尾部//1 讀入輸入文件的編碼格式，并按其編碼全部讀入文本      Encoding encode1 = fileEncode.GetFileEncodeType(xml_in_file);            StringBuilder strSb1 = new StringBuilder();      string strStr1 = File.ReadAllText(xml_in_file, encode1);//      StringBuilder strSb = new StringBuilder();      strSb.Clear();//2 讀入輸出文件的編碼格式，并按其編碼全部讀入文本      Encoding encode6 = fileEncode.GetFileEncodeType(xml_out_file);      strSb.AppendFormat("{0} /r/n", File.ReadAllText(xml_out_file, encode6));      strSb.Replace(strStr1, "");//舊的同名段落替換為空//新節點片段，替換整體末尾標簽，即是加入到末尾      strSb.Replace("＜/object_set＞", strStr1 + "/r/n" + "＜/object_set＞");//新的插入到末尾      // FileInfo myFile = new FileInfo(xml_out_file);      // StreamWriter sw = myFile.CreateText();      StreamWriter sw = new StreamWriter(xml_out_file, false, encode6);//Encoding.GetEncoding("GB2312"));      sw.Write(strSb.ToString());      sw.Close();    }  }//=2=獲得文件編碼格式的類  public class fileEncode  {//獲得文件編碼格式的類    public static System.Text.Encoding GetFileEncodeType(string filename)    {      System.IO.FileStream fs = new System.IO.FileStream(filename, System.IO.FileMode.Open, System.IO.FileAccess.Read);      System.IO.BinaryReader br = new System.IO.BinaryReader(fs);      Byte[] buffer = br.ReadBytes(2);      br.Close();      fs.Close();      if (buffer[0] >= 0xEF)      {        if (buffer[0] == 0xEF && buffer[1] == 0xBB)        {          return System.Text.Encoding.UTF8;        }        else if (buffer[0] == 0xFE && buffer[1] == 0xFF)        {          return System.Text.Encoding.BigEndianUnicode;        }        else if (buffer[0] == 0xFF && buffer[1] == 0xFE)        {          return System.Text.Encoding.Unicode;        }        else        {          return System.Text.Encoding.Default;        }      }      else      {        return System.Text.Encoding.Default;      }    }  }}