C# webBrowser操作 javascript

using System;
using System.Windows.Forms;

namespace Demo
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            webBrowser1.DocumentText = ("<select id=\"pid\" onchange=\"gradeChange()\">" +
                                        "<option grade=\"1\" value=\"a\">选项一</a>" +
                                        "<option grade=\"2\" value=\"b\">选项二</a>" +
                                        "</select>" +
                                        "<script type=\"text/javascript\">" +
                                        "function gradeChange(){" +
                                        "var objS = document.getElementById(\"pid\");" +
                                        "var grade = objS.options[objS.selectedIndex].grade;" +
                                        "alert(grade);" +
                                        "}" +
                                        "</script>");
        }

        private void button2_Click(object sender, EventArgs e)
        {
            HtmlElementCollection elements = webBrowser1.Document.GetElementById("pid").GetElementsByTagName("option");
            foreach (HtmlElement option in elements)
            {
                option.SetAttribute("selected", "selected");
                webBrowser1.Document.GetElementById("pid").RaiseEvent("onchange");
                break;
            }
        }
    }
}