在WatiN浏览器中自定义beforennavigate事件

我试图通过修改"before navigation"事件中的标题来使用WatiN框架更改用户代理字符串。我想我真的很接近一个解决方案，但由于某种原因，我被重定向到一个空白的页面，下面的代码在url被访问了一秒钟左右。

如果我注释掉自定义BeforeNavigate-events，浏览器不会重定向到空白页面，这使我相信我的自定义事件需要稍微修改。你知道是什么问题吗?

要重现这个问题，请下载WatiN并运行下面的代码:

private SHDocVw.InternetExplorerClass _ieInstance;
private const string UserAgent = 
   "User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64)" +
    "AppleWebKit/537.36 (KHTML, like Gecko) Chrome/35.0.1916.153" + 
    "Safari/537.36'r'n";
bool _renavigating;
private IE _browser;
private void Form1_Load(object sender, EventArgs e)
{
    _ieInstance = _ieInstance = new SHDocVw.InternetExplorerClass();
    //_ieInstance.BeforeNavigate2 += BeforeNavigate;
    _browser = new IE(_ieInstance);
    _browser.GoTo("http://www.procato.com/my+headers/");
}
void BeforeNavigate(object pDisp, ref object url, ref object flags, 
     ref object targetFrameName, ref object postData, ref object headers, 
     ref bool cancel)
{
    if (!string.IsNullOrEmpty(UserAgent))
    {
        if (!_renavigating)
        {
            headers += string.Format("User-Agent: {0}'r'n", UserAgent);
            _renavigating = true;
            cancel = true;
            _ieInstance.Navigate((string)url, 
                 (string)targetFrameName, (byte[])postData, (string)headers);
        }
        else
        {
            _renavigating = false;
        }
    }
}

---

@Noseratio的建议也给了我一个空白页:

readonly SynchronizationContext _sc = SynchronizationContext.Current;
void BeforeNavigate(object pDisp, ref object url, ref object flags, 
     ref object targetFrameName, ref object postData, ref object headers, 
     ref bool cancel)
{
    if (!string.IsNullOrEmpty(UserAgent))
    {
        if (!_renavigating)
        {
            var newHeaders = string.Format("User-Agent: {0}'r'n", UserAgent);
            _renavigating = true;
            cancel = true;
            string thatUrl = (string)url;
            object thatFlags = flags;
            object thatTargetFrameName = targetFrameName;
            object thatPostData = postData;
            object thatHeaders = (headers != null && headers != Type.Missing && 
                               headers != System.DBNull.Value) ?
                (headers.ToString() + newHeaders) : newHeaders;
            _sc.Post(_ => 
            { 
                ((SHDocVw.IWebBrowser)pDisp).Navigate(
                    thatUrl, 
                    ref thatFlags,
                    ref thatTargetFrameName,
                    ref thatPostData,
                    ref thatHeaders);
            }, null);
        }
        else
        {
            _renavigating = false;
        }
    }
}

private void Form1_Load(object sender, EventArgs e)
{
    _ieInstance = _ieInstance = new SHDocVw.InternetExplorerClass();
    _browser = new IE(_ieInstance);
    _ieInstance.Navigate("http://www.procato.com/my+headers/", Headers: UserAgent);
}