C#中HTMLDocument接口异常。

html页面代码为：<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml">
<head><title></title>
    <style type="text/css">
        .style1
        {
            width: 100%;
        }
        .style5
        {
            text-align: left;
            width: 330px;
            height: 123px;
        }
        .style6
        {
            width: 330px;
        }
        .style7
        {
            width: 166px;
        }
        .style8
        {
            width: 74px;
        }
        .style9
        {
            text-align: center;
            width: 333px;
            height: 123px;
        }
        .style10
        {
            width: 333px;
        }
        .style11
        {
            height: 123px;
        }
        .style12
        {
            width: 333px;
            height: 280px;
        }
        .style13
        {
            width: 330px;
            height: 280px;
        }
        .style14
        {
            height: 280px;
        }
    </style>
</head>
<body>
  
    <form method="post" action="login.aspx" id="form1">
<div class="aspNetHidden">
<input type="hidden" name="__VIEWSTATE" id="__VIEWSTATE" value="/wEPDwUJNTg4NDY2NjIzZGQRa1MztZzNYRAxcKVcBH9vAeeP51Fy6RcDNFJAkezD9Q==" />
</div><div class="aspNetHidden">    <input type="hidden" name="__EVENTVALIDATION" id="__EVENTVALIDATION" value="/wEWBAKq3a/nCALFppKhDQLlz6v5DALOmr/mDsLIrPNOtCeCNX8SWjZmF+oeFxzollBkynDveu/ZN3NW" />
</div>
    <table class="style1" width="1280" height="690" background="images/login_back.jpg">
        <tr>
            <td class="style9">
                <span id="Label1" style="font-size:XX-Large;font-family: 宋体, Arial, Helvetica, sans-serif">WEB系统待测登录页面</span>
            </td>
            <td class="style5">
                </td>
            <td class="style11">
                </td>
        </tr>
        <tr>
            <td class="style12">
                <table class="style1">
                    <tr>
                        <td class="style7">
                            &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 用户名：</td>
                        <td class="style8">
                            <input name="UserName_TextBox" type="text" id="UserName_TextBox" />
                        </td>
                    </tr>
                    <tr>
                        <td class="style7">
                            &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 密 码 ：</td>
                        <td class="style8">
                            <input name="PassWords_TextBox" type="text" id="PassWords_TextBox" />
                        </td>
                    </tr>
                    <tr>
                        <td colspan="2">
                            &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
                            <input type="submit" name="Login_Botton" value="登录" id="Login_Botton" />
                        &nbsp;&nbsp;&nbsp;
                            <span id="messageLabel" style="color:#CC0000;"></span>
                        </td>
                    </tr>
                </table>
            </td>
            <td class="style13">
            </td>
            <td class="style14">
                </td>
        </tr>
        <tr>
            <td class="style10">
                &nbsp;</td>
            <td class="style6">
                &nbsp;</td>
            <td>
                &nbsp;</td>
        </tr>
    </table>
    </form>
  
</body>
</html>C#代码：
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Diagnostics; 
using System.Threading; 
using SHDocVw; 
using mshtml;namespace TestProgram
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Launching IE ...");
            Process p = Process.Start("iexplore.exe", "about:blank");
            Thread.Sleep(3000);
            Console.WriteLine("Attaching to IE ...");
            InternetExplorer ie = null;
            if (p != null)
            {
                SHDocVw.ShellWindows allBrowser = new ShellWindows();
                if (allBrowser.Count != 0)
                {
                    for (int i = 0; i < allBrowser.Count; i++)
                    {
                        if (allBrowser.Item(i) is InternetExplorer)
                        {                            InternetExplorer e = (InternetExplorer)allBrowser.Item(i);                            if (e.HWND == (int)p.MainWindowHandle)
                            {
                                ie = e;
                                break;
                            }
                        }
                    }
                }
            }
            else
            {
                Console.WriteLine("Error: Can not Launch IE");
                return;
            }
            if (ie == null)
            {
                Console.WriteLine("Error: Can not attach to IE");
                return;
            }
            Console.WriteLine("Navigating ...");
          //  object o = String.Empty;
            object o = null;
            ie.Navigate("www.baidu.com", ref o, ref o, ref o, ref o);
            //ie.Navigate("www.baidu.com", ref o, ref o, ref o, ref o);
            Thread.Sleep(5000);
            //得到一个Text Box 
            Console.WriteLine("Inputing Keyword ...");
            HTMLDocument doc = (HTMLDocument)ie.Document;
            HTMLInputElement keyword = (HTMLInputElement)doc.getElementById("kw");
            keyword.value = "天津";
            Thread.Sleep(5000);
            //验证 
            if (keyword.getAttribute("size", 0).ToString().Equals("36"))
                Console.WriteLine("Validation Passed! Size is Correct");
            else
                Console.WriteLine("Validation Failed! Size is wrong");
            //得到一个按钮 
            Console.WriteLine("Clicking Submit ...");
            HTMLInputElement submit = (HTMLInputElement)doc.getElementById("su");
            submit.click();
            Thread.Sleep(4000);
            //验证 
            if (string.Equals(doc.title.Trim(), "百度搜索_天津"))
            {
                Console.WriteLine("Validation Passed! Title is Corrected");
                //Console.ReadLine; 
            }
            else
                Console.WriteLine("Validation Failed! Title is wrong");
            if (doc.body.innerText.Contains("天津地图"))
            {
                Console.WriteLine("Validation Passed! Body contains your string");
                Console.ReadLine();
            }
            else
                Console.WriteLine("Validation Failed! Body do not contain");
            ////得到一个链接 
            //Console.WriteLine("Clicking Login Button ..."); 
            //IHTMLElement userPanel = doc.getElementById("u"); 
            //IHTMLElementCollection HyperLinks = ((IHTMLElement2)userPanel).getElementsByTagName("a"); 
            //IHTMLElement login = (IHTMLElement)HyperLinks.item(null, 0); 
            //login.click(); 
        }
提示HTMLDocument doc = (HTMLDocument)ie.Document;HTMLDocument未知接口, ie.Navigate("http://localhost/TestWebsite/login.aspx", ref o, ref o, ref o, ref o);网址改为百度是可以的，求高手给予解答