如何读取、修改文本文件

using System;
using System.Collections;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Web;
using System.Web.SessionState;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.HtmlControls;
using System.Text;
using System.IO;
namespace CommonFunction
{
/// <summary>
/// operTxt 的摘要说明。
/// </summary>
public class operTxt : System.Web.UI.Page
{
protected System.Web.UI.WebControls.TextBox txtContent;
protected System.Web.UI.WebControls.Button btnRead;
protected System.Web.UI.HtmlControls.HtmlInputFile filePath;
protected string txtFilePath,fileExtName;
protected System.Web.UI.WebControls.Button btnRepaire;

private void Page_Load(object sender, System.EventArgs e)
{
// 在此处放置用户代码以初始化页面
}

#region Web Form Designer generated code
override protected void OnInit(EventArgs e)
{
//
// CODEGEN：该调用是 ASP.NET Web 窗体设计器所必需的。
//
InitializeComponent();
base.OnInit(e);
}

/// <summary>
/// 设计器支持所需的方法 - 不要使用代码编辑器修改
/// 此方法的内容。
/// </summary>
private void InitializeComponent()
{
this.btnRead.Click += new System.EventHandler(this.btnRead_Click);
this.btnRepaire.Click += new System.EventHandler(this.btnRepaire_Click);
this.Load += new System.EventHandler(this.Page_Load);

}
#endregion

private void btnRead_Click(object sender, System.EventArgs e)
{
if(filePath.PostedFile.FileName != "")
{
txtFilePath =filePath.PostedFile.FileName;
fileExtName = txtFilePath.Substring(txtFilePath.LastIndexOf(".")+1,3);

if(fileExtName !="txt" && fileExtName != "TXT")
{
Response.Write("请选择文本文件");
}
else
{
StreamReader fileStream = new StreamReader(txtFilePath,Encoding.Default);
txtContent.Text = fileStream.ReadToEnd();
fileStream.Close();
}
}
}

private void btnRepaire_Click(object sender, System.EventArgs e)
{
if(filePath.PostedFile.FileName != "")
{
txtFilePath =filePath.PostedFile.FileName;
fileExtName = txtFilePath.Substring(txtFilePath.LastIndexOf(".")+1,3);

if(fileExtName !="txt" && fileExtName != "TXT")
{
Response.Write("请选择文本文件");
}
else
{
StreamWriter fileWrite = new StreamWriter(txtFilePath,false,Encoding.Default);
fileWrite.Write(txtContent.Text);
fileWrite.Close();

}
}
}

}
}