How can you verify validity of an HTTPS/SSL certificate in .NET?

Question

How can you verify validity of an HTTPS/SSL certificate in .NET?

Ideally I want to establish an HTTPS connection to a website and then find out if that was done in a valid way (certificate not expired, host name matches, certificate chain trusted etc), but the built in HTTP Client seems to ignore certificate errors (and I'm not sure that physically downloading a web page is necessary to verify a certificate?).

I've tried to use the code below (adapted from an answer in the comments) but the ValidationCallback never gets called:

    static void Main(string[] args)
    {
        String url = "https://www.example.com";
        HttpWebRequest request = WebRequest.CreateHttp(url);
        request.GetResponse();
        request.ServerCertificateValidationCallback += ServerCertificateValidationCallback;
        Console.WriteLine("End");
        Console.ReadKey();
    }

    private static bool ServerCertificateValidationCallback(object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
    {
        if (sslPolicyErrors == SslPolicyErrors.None)
        {
            Console.WriteLine("Certificate OK");
            return true;
        }
        else
        {
            Console.WriteLine("Certificate ERROR");
            return false;
        }
    }

Answer 1

It doesn't get called because you're setting the ValidationCallback after you've already made the request.

Change it to this:

HttpWebRequest request = WebRequest.CreateHttp( url );
request.ServerCertificateValidationCallback += ServerCertificateValidationCallback;
using( HttpWebResponse response = (HttpWebResponse)request.GetResponse() ) { }
Console.WriteLine("End");

...and it will work.