Using WebBrowser in WPF

WebBrowser.cs

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// Using WebBrowser for Crowling in WPF
----------------------------------------------------
<WebBrowser Cursor="Arrow" Name="MyBrowser" LoadCompleted="MyBrowser_OnLoadCompleted" />
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// useful methods of WebBrowser
----------------------------------------------------
MyBrowser.Navigate(new Uri("http://google.com"));
 
private void MyBrowser_OnLoadCompleted(object sender,NavigationEventArgs e)
{
        ...
        // return source of loaded page in WebBrowser
        var document=(IHTMLDocument3) MyBrowser.Document;
 
        //Get Element and set its value
        document.getElementById("userName").setAttribute("value","myusername");
       
        //get Button on page and fire its click event
        document.getElementById("btnSubmit").click();
 
        // Invoke javascript function on page loaded on WebBrowser
        MyBrowser.InvokeScript("submitform",param1,param2,...);
 
        //get data from table in page
         _innerHtmldata =
                ((IHTMLDocument3) MyBrowser.Document).getElementById("datatable")
                                                     .innerHTML;
 
}

i get error on IHTMLDocument3
could not be found
????? solution???