webBrowser自动登陆问题

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;

namespace WindowsApplication3
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void webBrowser1_DocumentCompleted(object sender, WebBrowserDocumentCompletedEventArgs e)
        {
            HtmlElement btnSubmit = webBrowser1.Document.All["Submit"];
            HtmlElement tbUserid = webBrowser1.Document.All["username"];
            HtmlElement tbPasswd = webBrowser1.Document.All["password"];

            if (tbUserid == null || tbPasswd == null || btnSubmit == null)
                return;

            tbUserid.SetAttribute("value", "cwchyt");
            tbPasswd.SetAttribute("value", "75011");
            btnSubmit.InvokeMember("click");
            return;
        }

        string url = "";

        private void button1_Click(object sender, EventArgs e)
        {
            //循环执行
            for (int i = 0; i < 2; i++)
            {
                if (i == 0)
                {

                    url = "http://www.hzxsjy.com/blog/login.asp";
                    this.webBrowser1.DocumentCompleted += new WebBrowserDocumentCompletedEventHandler(webBrowser1_DocumentCompleted);
                    webBrowser1.Navigate(url);
                }
                if (i == 1)
                {
                    url = "http://www.szxq.com/blog/login.asp";
                    this.webBrowser1.DocumentCompleted += new WebBrowserDocumentCompletedEventHandler(webBrowser1_DocumentCompleted);
                    webBrowser1.Navigate(url);
                }
            }
        }
    }
}