关于禁止网站浏览的问题（是用SHDocVw）

try
            {                
                foreach (SHDocVw.InternetExplorer ie in new SHDocVw.ShellWindowsClass())
                {
                    bool limit = false;
                    ie.BeforeNavigate2 += new SHDocVw.DWebBrowserEvents2_BeforeNavigate2EventHandler(OnBeforeNavigate2);                    if (windowslimit.dsUnlimitUrl != null && windowslimit.dsUnlimitUrl.Tables.Count > 0 && windowslimit.dsUnlimitUrl.Tables[0].Rows.Count > 0)
                    {
                        foreach (DataRow dr in windowslimit.dsUnlimitUrl.Tables[0].Rows)
                        {
                            if (ie.LocationURL.ToString().IndexOf(dr[1].ToString()) > 0)
                            {
                                limit = true;
                            }
                        }
                    }
                    if (!limit)
                    {
                        ie.Quit();
                    }                    
                }
                
               
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            } protected virtual void OnBeforeNavigate2(object pDisp, ref object URL, ref object Flags, ref object TargetFrameName, ref object PostData, ref object Headers, ref bool Cancel)
        {     
        }打开网页后，过一会儿就会报错说是 “COM 对象与其基础 RCW 分开后不能再使用”
这是怎么回事，，求教各位大侠了，