.net怎样删除.txt文件中第一行数据

先读取，然后删除第一行，然后保存回去。
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using System;
using System.Collections;
using System.Collections.Generic;
using System.Configuration;
using System.Data;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;

using System.IO;
using System.Text;

public partial class line_Default : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
string filepath = Server.MapPath("1.txt");
DeleteLine(filepath);
}

private void DeleteLine(string filepath)
{
StringBuilder sb = new StringBuilder();
using (StreamReader sr = new StreamReader(filepath, Encoding.Default))
{
int i = 0;
string line;
wh