using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Data.SqlClient;
using System.Text;
using

public partial class WebForm8 : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {

    }
    protected void LinkButton1_Click(object sender, EventArgs e)
    {
        int id;
        DataSet data =new DataSet();
        StreamReader sr = new StreamReader(Server.MapPath("leave.xml"));
        StreamWriter sw;
        DataTable dt = new DataTable();
        DataRow dr;
        string str;
        str = sr.ReadingLine();
        data.ReadXml(sr);
        sr.Close();
        dt = data.Tables["leave"];
        dr = dt.NewRow();
        if (dt.Rows.Count == 0) id = 1;
        else
            id = dt.Rows.Count + 1;
        dr["UserId"] = id;
        dr["UserName"] = TextBox1.Text;
        dr["Email"] = TextBox2.Text;
        dr["PostDate"] = DateTime.Now;
        dr["Words"] = TextBox3.Text;
        dt.Rows.Add(dr);
        sw = new StreamWriter(Server.MapPath("leave.xml"),false);
        sw.WriteLine(str);
        sw.Close();
        sw = new StreamWriter(Server.MapPath("leave.xml"),true);
        data.WriterXml(sw);
        sw.Close();
        TextBox1.Text = "";
        TextBox2.Text = "";
        TextBox3.Text = "";
        Response.Write("\t\t留言已被保存");

        string name = TextBox1.Text;
        string email = TextBox2.Text;
        SqlConnection con = new SqlConnection("server=C29\\SQLEXPRESS;database=reg;UID=sa;PWD=sa;");
        string str = "select * from [reg_1] where 姓名='" + TextBox1.Text + "'and  电子邮件='" + TextBox2.Text + "'";
        con.Open(); //   打开连接
        SqlCommand cmd = new SqlCommand(str, con);   //   创建Command对象
        SqlDataReader dr = cmd.ExecuteReader(); //   执行ExecuteReader()方法
        if (dr.Read())
        {
            Response.Write("<script>alert('留言成功');location='WebForm8.aspx';</script>");
        }
        else
        {
            Response.Write("<script>alert('提交失败！');location='WebForm8.aspx';</script>");
        }
    }
    protected void LinkButton2_Click(object sender, EventArgs e)
    {
        Response.Redirect("WebForm6.aspx");
    }
    protected void LinkButton3_Click(object sender, EventArgs e)
    {
        Response.Redirect("WebForm9.aspx");
    }
    protected void TextBox1_TextChanged(object sender, EventArgs e)
    {

    }
}
6个错误如下：
“”并不包含“ReadingLine”的定义
“System.Data.DataSet”并不包含“WriterXml”的定义
已在此范围定义了名为“str”的局部变量。
已在此范围定义了名为“dr”的局部变量
无法将类型“System.Data.SqlClient.SqlDataReader”隐式转换为“System.Data.DataRow”
“System.Data.DataRow”并不包含“Read”的定义

不包含，说明当前类里不存在你写的函数！

SqlDataReader dr = cmd.ExecuteReader();
DataRow dr;
string str;
string str = "select * from [reg_1] where 姓名='" + TextBox1.Text + "'and  电子邮件='" + TextBox2.Text + "'";
你重复定义肯定出错啦!