c＃为什么从未调用SSL ServerCertificateValidationCallback？

Question

为什么这段代码永远不会执行ServerCertificateValidationCallback事件处理程序？

ServicePointManager.MaxServicePointIdleTime = 0;

try
{
    String url = "https://" + Domain;
    HttpWebRequest request = WebRequest.CreateHttp(url);
    request.Timeout = 10000;
    request.KeepAlive = false;
    request.AllowAutoRedirect = true;
    request.ProtocolVersion = HttpVersion.Version11;
    request.ReadWriteTimeout = 10000;
    request.Accept = "text/html, application/xhtml+xml, application/xml; q=0.9, image/webp, */*; q=0.8";
request.UserAgent = "Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/48.0.2564.97 Safari/537.36";
    request.GetResponse();
    request.ServerCertificateValidationCallback += delegate (object sender, X509Certificate cert, X509Chain chain, SslPolicyErrors sslError) {

        Console.WriteLine("CALLED!");

        return false;
    };
}
catch
{
    Console.WriteLine("ERROR!");
}