WebBrowser 获取HttpOnly的Cookie

private const int INTERNET_COOKIE_HTTPONLY = 0x00002000;

[DllImport("wininet.dll", SetLastError = true)]
private static extern bool InternetGetCookieEx(
    string url,
    string cookieName,
    StringBuilder cookieData,
    ref int size,
    int flags,
    IntPtr pReserved);
public static string GetCookie(string url)
{
    int size = 512;
    StringBuilder sb = new StringBuilder(size);
    if (!InternetGetCookieEx(url, null, sb, ref size, INTERNET_COOKIE_HTTPONLY, IntPtr.Zero))
    {
        if (size < 0)
        {
            return null;
        }
        sb = new StringBuilder(size);
        if (!InternetGetCookieEx(url, null, sb, ref size, INTERNET_COOKIE_HTTPONLY, IntPtr.Zero))
        {
            return null;
        }
    }
    return sb.ToString();
}