Encoding encode=Encoding.GetEncoding("GB2312"));
 可以使用后附的类，先读文件的编码格式
 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=按文件编码格式读写
  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";//输入片段，其它程序生成的是一个节点
      string xml_out_file="c:\\all.xml";//整体，新节点片段，要追加到其尾部
//1 读入输入文件的编码格式，并按其编码全部读入文本
      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 读入输出文件的编码格式，并按其编码全部读入文本
      Encoding encode6 = fileEncode.GetFileEncodeType(xml_out_file);
      strSb.AppendFormat("{0} \r\n", File.ReadAllText(xml_out_file, encode6));
      strSb.Replace(strStr1, "");//旧的同名段落替换为空
//新节点片段，替换整体末尾标签，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=获得文件编码格式的类
  public class fileEncode
  {//获得文件编码格式的类
    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;
      }
    }

  }
}