backgroundWorker线程问题

我在backgroundWorker的DoWork里面执行了以下代码结果就报错说“当前线程不在单线程单元中，因此无法实例化 ActiveX 控件“8856f961-340a-11d0-a96b-00c04fd705a2”
代码如下：
 try
            {
                string allHtml = "";
                HttpWebRequest webrequest = (HttpWebRequest)WebRequest.Create(LoginUrl);
                webrequest.Timeout = 30000;
                webrequest.CookieContainer = cookieCon;
                HttpWebResponse myresponse = (HttpWebResponse)webrequest.GetResponse();
                Stream streamResponse = myresponse.GetResponseStream();
                StreamReader streamRead = new StreamReader(streamResponse, Encoding.GetEncoding(Encode));
                allHtml = streamRead.ReadToEnd();
                streamResponse.Close();
                streamRead.Close();
                //转换HtmlDocument
                WebBrowser ber = new WebBrowser(); //错误在这里
                ber.Navigate("about:blank");
                ber.DocumentText = "<html><body>this is a htmlbr/><input type='text' name='username'/><br/></body></html>";
               HtmlDocument doc = ber.Document;
                doc.Write(allHtml); 
            }
            catch (Exception)
            {
            }
应该是UI线程问题吧 怎么解决或者还有其他办法获取对应URl的HtmlDocument对象吗？