title: How to call default ServerCertificateValidationCallback inside customized validation?
body: <p>I want to modify .Net's default <code>ServerCertificateValidationCallback</code> to validate as true some of my company's certificates, but keeping the default validation for other certificates. </p>
  
  <p>I can't seem to do this since the default <code>ServerCertificateValidationCallback</code> value is null.</p>
  
  <pre><code>ServicePointManager.ServerCertificateValidationCallback = 
  (sender, certificate, chain, sslPolicyErrors) =&gt; 
   validCertificatesSerialNumbers.Contains(certificate.GetSerialNumberString()) ||    
   defaultlCallback.Invoke(sender, certificate, chain, sslPolicyErrors) //How do I set defaultCallback?
  ;
  </code></pre>
  
  <p>Thank you</p>