报错指示如下：
Error 1 'System.Windows.Controls.WebBrowser' does not contain a definition for 'DocumentStream' and no extension method 'DocumentStream' accepting a first argument of type 'System.Windows.Controls.WebBrowser' could be found (are you missing a using directive or an assembly reference?)

我找了一晚上也没有找到原因在哪，好像也没有落下什么没有引入文件，在百度谷歌上找了一晚也没有找到解答，敬请高手们帮忙：
代码如下：
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Shapes;
using System.Runtime.InteropServices;
using System.Windows.Forms;

namespace JobOnePlusOne
{
     public partial class GetPIN : Window
    {
        string gethtml = "";
        public GetPIN(string openURL)
        {
            InitializeComponent();
            myBrower.Navigate(openURL);
        }

        private void Window_Closing(object sender, System.ComponentModel.CancelEventArgs e)
        {
            gethtml = gethtml + "1";

        }
        private void myBrower_DocumentCompleted(object sender, WebBrowserDocumentCompletedEventArgs e)
        {
            if (this.myBrower.Document !=null)
            {
                System.IO.StreamReader getReader = new System.IO.StreamReader(myBrower.DocumentStream, System.Text.Encoding.GetEncoding("gb2312"));
                gethtml = getReader.ReadToEnd();
            }
        }
    }
}